Hi simflex,
Please refer below sample.
HTML
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajax" %>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<ajax:ModalPopupExtender ID="installAddress_MPE" runat="server" TargetControlID="lnkFake"
    PopupControlID="installnew" CancelControlID="btnClose" BackgroundCssClass="modalBackground popup" />
<asp:LinkButton ID="lnkFake" Text="" runat="server" />
<asp:Panel ID="installnew" runat="server" CssClass="modalPopup" align="center" Style="display: none;">
    <asp:Table ID="Table4" class="popupdiv" Style="background-color: white; font-size: 12pt;
        color: Black; width: 900px;" runat="server">
        <asp:TableRow>
            <asp:TableCell ID="TableCell3" Style="background-color: green; font-size: 12pt; color: White;"
                runat="server" ColumnSpan="6">
                <asp:Label ID="installAddressHeader" runat="server" Text="New Installation Address Entry" />
            </asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableHeaderCell CssClass="align-right"> </asp:TableHeaderCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableCell ID="TableCell1" Style="background-color: olive; font-size: 12pt; color: White;"
                runat="server" ColumnSpan="6">
                <asp:Label ID="Label1" runat="server" Style="float: left; width: 100%; text-align: center;
                    font-weight: bold !important;" Text="PROPERTY ADDRESS INFORMATION" />
            </asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableHeaderCell CssClass="align-right">Install Address:</asp:TableHeaderCell>
            <asp:TableCell ColumnSpan="2">
                <asp:TextBox ID="instAddress" runat="server" /></asp:TableCell>
            <asp:TableHeaderCell CssClass="align-right">Water Account #:</asp:TableHeaderCell>
            <asp:TableCell ColumnSpan="2">
                <asp:TextBox ID="instAddressAccount" runat="server" /></asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableHeaderCell CssClass="align-right">City:</asp:TableHeaderCell>
            <asp:TableCell>
                <asp:DropDownList ID="instAddressCity" AutoPostBack="true" runat="server" />
                <%--OnSelectedIndexChanged="instAddressCity_Changed"--%>
            </asp:TableCell>
            <asp:TableHeaderCell CssClass="align-left">State:</asp:TableHeaderCell>
            <asp:TableCell>
                <asp:Label ID="instAddressState" runat="server" Text="GA" /></asp:TableCell>
            <asp:TableHeaderCell CssClass="align-left">Zip:</asp:TableHeaderCell>
            <asp:TableCell>
                <asp:DropDownList ID="instAddressZip" runat="server" />
            </asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableHeaderCell ColumnSpan="5" CssClass="align-left">Install address same as mailing address?</asp:TableHeaderCell>
            <asp:TableCell>
                <asp:CheckBox ID="SameAsMailing" runat="server" /></asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableHeaderCell ColumnSpan="3" CssClass="align-left">Address:</asp:TableHeaderCell>
            <asp:TableCell ColumnSpan="3">
                <asp:TextBox ID="mailStreetAddress" runat="server" /></asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableHeaderCell CssClass="align-left">City:</asp:TableHeaderCell>
            <asp:TableCell>
                <asp:TextBox ID="mailCity" runat="server" /></asp:TableCell>
            <asp:TableHeaderCell CssClass="align-left">State:</asp:TableHeaderCell>
            <asp:TableCell>
                <asp:TextBox ID="mailState" runat="server" /></asp:TableCell>
            <asp:TableHeaderCell CssClass="align-left">Zip:</asp:TableHeaderCell>
            <asp:TableCell>
                <asp:TextBox ID="mailZip" runat="server" /></asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableHeaderCell CssClass="align-left">Total toilets:</asp:TableHeaderCell>
            <asp:TableCell ColumnSpan="2">
                <asp:DropDownList ID="instAddressToilets" runat="server">
                    <asp:ListItem Selected="True" Text="0" Value="0" />
                    <asp:ListItem Text="1" Value="1" />
                    <asp:ListItem Text="2" Value="2" />
                    <asp:ListItem Text="3" Value="3" />
                    <asp:ListItem Text="4" Value="4" />
                    <asp:ListItem Text="5" Value="5" />
                    <asp:ListItem Text="6" Value="6" />
                    <asp:ListItem Text="7" Value="7" />
                    <asp:ListItem Text="8" Value="8" />
                </asp:DropDownList>
            </asp:TableCell>
            <asp:TableHeaderCell CssClass="align-right">Year Built:</asp:TableHeaderCell>
            <asp:TableCell ColumnSpan="2">
                <asp:TextBox ID="instYearBuilt" runat="server" /></asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableHeaderCell CssClass="align-right"> </asp:TableHeaderCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableCell ID="TableCell2" Style="background-color: olive; font-size: 12pt; color: White;"
                runat="server" ColumnSpan="6">
                <asp:Label ID="Label3" Style="float: left; width: 100%; text-align: center; font-weight: bold !important;"
                    runat="server" Text="OWNER / APPLICANT INFORMATION" />
            </asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableCell ColumnSpan="4" Style="text-align: left;">
                <asp:Label ID="dataTest" runat="server" />
            </asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableCell Style="white-space: nowrap">Primary First Name:</asp:TableCell>
            <asp:TableCell>
                <asp:TextBox ID="primaryFirstName" runat="server" /></asp:TableCell>
            <asp:TableCell Style="white-space: nowrap">Primary Last Name:</asp:TableCell>
            <asp:TableCell>
                <asp:TextBox ID="primaryLastName" runat="server" /></asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableCell Style="white-space: nowrap">Secondary First Name:</asp:TableCell>
            <asp:TableCell>
                <asp:TextBox ID="secondaryFirstName" runat="server" /></asp:TableCell>
            <asp:TableCell Style="white-space: nowrap">Secondary Last Name:</asp:TableCell>
            <asp:TableCell>
                <asp:TextBox ID="secondaryLastName" runat="server" /></asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableCell> </asp:TableCell>
            <asp:TableCell>Modifier:</asp:TableCell>
            <asp:TableCell>
                <asp:DropDownList ID="ownerModifier" runat="server">
                    <asp:ListItem Selected="True" Text="None" Value="0" />
                    <asp:ListItem Text="And" Value="And" />
                    <asp:ListItem Text="Or" Value="Or" />
                </asp:DropDownList>
            </asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableCell>Primary Phone:</asp:TableCell>
            <asp:TableCell>
                <asp:TextBox ID="phoneNumber" runat="server" />
                <ajax:MaskedEditExtender ID="phoneNumberMask" runat="server" TargetControlID="phoneNumber"
                    ClearMaskOnLostFocus="false" Mask="(999) 999-9999" MaskType="Number" />
            </asp:TableCell>
            <asp:TableCell>Alternate Phone:</asp:TableCell>
            <asp:TableCell>
                <asp:TextBox ID="altPhoneNumber" runat="server" />
                <ajax:MaskedEditExtender ID="altPhoneNumberMask" runat="server" TargetControlID="altPhoneNumber"
                    ClearMaskOnLostFocus="false" Mask="(999) 999-9999" MaskType="Number" />
            </asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableCell>Email:</asp:TableCell>
            <asp:TableCell>
                <asp:TextBox ID="emailAddress" runat="server" />
                <asp:RegularExpressionValidator ID="emailRegEx" runat="server" ControlToValidate="emailAddress"
                    Text="*!*" ErrorMessage="A valid Email Address must be supplied for Owner/Applicant Email Address"
                    ValidationExpression="^(['_a-z0-9-]+)(\.['_a-z0-9-]+)*@([a-z0-9-]+)(\.[a-z0-9-]+)*(\.[a-z]{2,5})$" />
            </asp:TableCell>
            <asp:TableCell>Business:</asp:TableCell>
            <asp:TableCell>
                <asp:CheckBox ID="isBusiness" runat="server" Checked="true" Enabled="false" Visible="false" /></asp:TableCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableHeaderCell CssClass="align-right"> </asp:TableHeaderCell>
        </asp:TableRow>
        <asp:TableRow>
            <asp:TableCell ID="TableCell4" Style="background-color: olive; font-size: 12pt; color: White;"
                runat="server" ColumnSpan="6">
                <asp:Label ID="Label4" runat="server" Style="float: left; width: 100%; text-align: center;
                    font-weight: bold !important;" Text="TOILET INFORMATION" />
            </asp:TableCell>
        </asp:TableRow>
    </asp:Table>
    <div id="divRebatable" class="popupdiv">
        <br />
        <div id="newtoilet">
            <asp:TextBox ID="txt_toiletsizes" placeholder="Enter new Toiltet GPF to dropdown"
                runat="server"></asp:TextBox>
            <asp:TextBox ID="txt_newAmount" placeholder="Enter amount" Width="100px" runat="server"></asp:TextBox>
            <asp:Button ID="btn_add" runat="server" Text="Add It" /></div>
        <asp:CheckBox ID="chkAdd" runat="server" Text="Add new GPF" /><br />
        <br />
        <asp:Table ID="rebateTable" runat="server">
            <asp:TableRow ID="allToilets" runat="server">
                <asp:TableHeaderCell>How many toilets did you upgrade?</asp:TableHeaderCell>
                <asp:TableCell>
                    <asp:DropDownList ID="ddlNumber" runat="server" AutoPostBack="true">
                        <%--OnSelectedIndexChanged="ddlNumber_SelectedIndexChanged"--%>
                        <asp:ListItem Text="--Select--" Value=""></asp:ListItem>
                        <asp:ListItem Text="1" Value="1"></asp:ListItem>
                        <asp:ListItem Text="2" Value="2"></asp:ListItem>
                        <asp:ListItem Text="3" Value="3"></asp:ListItem>
                    </asp:DropDownList>
                </asp:TableCell>
            </asp:TableRow>
        </asp:Table>
    </div>
    <br />
    <div class="popupdiv">
        <asp:Repeater ID="DynamicRepeater" runat="server">
            <%--OnItemDataBound="OnItemDataBound"--%>
            <HeaderTemplate>
                <table border="1">
                    <tr>
                        <th>
                            Toilet GPF
                        </th>
                        <th>
                            Toilet<br />
                            Model #
                        </th>
                        <th>
                            Tank Model #
                        </th>
                        <th>
                            Bowl Model#
                        </th>
                        <th>
                            Purchased<br />
                            From
                        </th>
                        <th>
                            Purchase<br />
                            Date
                        </th>
                        <th>
                            Plumber/<br />
                            Installer
                        </th>
                        <th>
                            Installation<br />
                            Date
                        </th>
                        <th>
                            Date<br />
                            Upgraded
                        </th>
                    </tr>
            </HeaderTemplate>
            <ItemTemplate>
                <tr>
                    <td>
                        <asp:DropDownList ID="ddlToiletGPF" AutoPostBack="true" runat="server">
                            <%--OnSelectedIndexChanged="OnSelectedIndexChanged"--%>
                        </asp:DropDownList>
                    </td>
                    <td>
                        <asp:TextBox ID="ToiletModelNumber" Style="width: 100%" runat="server"></asp:TextBox>
                    </td>
                    <td>
                        <asp:TextBox ID="TankModelNumber" Style="width: 100%" runat="server"></asp:TextBox>
                    </td>
                    <td>
                        <asp:TextBox ID="BowlmodelNumber" Style="width: 100%" runat="server"></asp:TextBox>
                    </td>
                    <td>
                        <asp:TextBox ID="PurchasedFrom" Style="width: 100%" runat="server"></asp:TextBox>
                    </td>
                    <td>
                        <asp:TextBox ID="PurchaseDate" Style="width: 100%" runat="server"></asp:TextBox>
                    </td>
                    <td>
                        <asp:TextBox ID="Installer" Style="width: 100%" runat="server"></asp:TextBox>
                    </td>
                    <td>
                        <asp:TextBox ID="InstallationDate" Style="width: 100%" runat="server"></asp:TextBox>
                    </td>
                    <td>
                        <asp:TextBox ID="DateUpgraded" Style="width: 100%" runat="server"></asp:TextBox>
                    </td>
                </tr>
            </ItemTemplate>
            <FooterTemplate>
                </table>
            </FooterTemplate>
        </asp:Repeater>
        <br />
        <asp:Label ID="lblSuccess" runat="server" Text=""></asp:Label>
    </div>
    <asp:Button ID="btnClose" runat="server" Text="Cancel" />
</asp:Panel>
<asp:TableRow>
    <asp:TableCell Width="100%" HorizontalAlign="Center" ColumnSpan="6">
        Enter search term:
        <asp:TextBox ID="txtSearch" runat="server" />
        <asp:Button ID="btnSearch" Text="Search" runat="server" OnClick="OnSearch" />
         
        <asp:LinkButton ID="showTips" runat="server" Text="Search Tips" />
        <div id="divBackground">
        </div>
        <asp:HiddenField ID="hfCountry" runat="server" />
    </asp:TableCell>
</asp:TableRow>
<asp:HiddenField ID="hfIsExist" runat="server" />
<link href="https://ajax.aspnetcdn.com/ajax/jquery.ui/1.9.2/themes/blitzer/jquery-ui.css"
    rel="Stylesheet" type="text/css" />
<script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.10.0.min.js" type="text/javascript"></script>
<script src="https://ajax.aspnetcdn.com/ajax/jquery.ui/1.9.2/jquery-ui.min.js" type="text/javascript"></script>
<script type="text/javascript">
    $(function () {
        $("[id$=txtSearch]").autocomplete({
            source: function (request, response) {
                $.ajax({
                    url: '<%=ResolveUrl("~/CS.aspx/GetCustomers") %>',
                    data: "{ 'prefix': '" + request.term + "'}",
                    dataType: "json",
                    type: "POST",
                    contentType: "application/json; charset=utf-8",
                    success: function (data) {
                        if (data.d.length > 0) {
                            response($.map(data.d, function (item) {
                                return {
                                    label: item.split('-')[0],
                                    val: item.split('-')[1]
                                }
                            }))
                            $("[id$=hfIsExist]").val('true');
                        }
                        else {
                            $("[id$=hfIsExist]").val('false');
                        }
                    },
                    error: function (response) {
                        alert(response.responseText);
                    }
                });
            },
            select: function (e, i) {
                $("[id$=hfCountry]").val(i.item.val);
                $("[id$=hfIsExist]").val('true');
            },
            minLength: 1
        });
    });  
</script>
 
 
Namespaces
C#
using System.Data;
using System.Configuration;
using System.Web.Services;
using System.Data.SqlClient;
VB.Net
Imports System.Data
Imports System.Web.Services
Imports System.Configuration
Imports System.Data.SqlClient
Code
C#
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindRepeater();
    }
}
[WebMethod()]
public static string[] GetCustomers(string prefix)
{
    List<string> fillAddress = new List<string>();
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "select Name, Country from Customers where Name like @SearchText + '%'";
            cmd.Parameters.AddWithValue("@SearchText", prefix);
            cmd.Connection = conn;
            conn.Open();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                    fillAddress.Add(string.Format("{0}-{1}", sdr["Name"], sdr["Country"]));
            }
            conn.Close();
        }
    }
    return fillAddress.ToArray();
}
protected void OnSearch(object sender, EventArgs e)
{
    if (hfIsExist.Value != "true")
    {
        installAddress_MPE.Show();
    }
    else
    {
        installAddress_MPE.Hide();
    }
}
private void BindRepeater()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[3]{
                    new DataColumn("Toilet Size"),
                    new DataColumn("Model #"),
                    new DataColumn("Date Upgraded")
});
    DropDownList ddl = DynamicRepeater.FindControl("ddlToiletGPF") as DropDownList;
    TextBox txt = DynamicRepeater.FindControl("ModelNumber") as TextBox;
    TextBox txt2 = DynamicRepeater.FindControl("DateUpgraded") as TextBox;
    dt.Rows.Add(ddl, txt, txt2);
    DynamicRepeater.DataSource = dt;
    DynamicRepeater.DataBind();
}
VB.Net
Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Me.BindRepeater()
    End If
End Sub
<WebMethod()>
Public Shared Function GetCustomers(ByVal prefix As String) As String()
    Dim fillAddress As List(Of String) = New List(Of String)()
    Using conn As SqlConnection = New SqlConnection()
        conn.ConnectionString = ConfigurationManager.ConnectionStrings("constr").ConnectionString
        Using cmd As SqlCommand = New SqlCommand()
            cmd.CommandText = "select Name, Country from Customers where Name like @SearchText + '%'"
            cmd.Parameters.AddWithValue("@SearchText", prefix)
            cmd.Connection = conn
            conn.Open()
            Using sdr As SqlDataReader = cmd.ExecuteReader()
                While sdr.Read()
                    fillAddress.Add(String.Format("{0}-{1}", sdr("Name"), sdr("Country")))
                End While
            End Using
            conn.Close()
        End Using
    End Using
    Return fillAddress.ToArray()
End Function
Protected Sub OnSearch(ByVal sender As Object, ByVal e As EventArgs)
    If hfIsExist.Value <> "true" Then
        installAddress_MPE.Show()
    Else
        installAddress_MPE.Hide()
    End If
End Sub
Private Sub BindRepeater()
    Dim dt As DataTable = New DataTable()
    dt.Columns.AddRange(New DataColumn(2) {New DataColumn("Toilet Size"), New DataColumn("Model #"), New DataColumn("Date Upgraded")})
    Dim ddl As DropDownList = TryCast(DynamicRepeater.FindControl("ddlToiletGPF"), DropDownList)
    Dim txt As TextBox = TryCast(DynamicRepeater.FindControl("ModelNumber"), TextBox)
    Dim txt2 As TextBox = TryCast(DynamicRepeater.FindControl("DateUpgraded"), TextBox)
    dt.Rows.Add(ddl, txt, txt2)
    DynamicRepeater.DataSource = dt
    DynamicRepeater.DataBind()
End Sub