Hi @safeer ,
I have made a small snippet for you.
Try this.
Html:-
<form id="form1" runat="server">
<cc1:ToolkitScriptManager runat="server">
</cc1:ToolkitScriptManager>
<div>
Name:<asp:TextBox ID="txtName" runat="server"></asp:TextBox>
<br />
City:<asp:TextBox ID="txtCity" runat="server"></asp:TextBox>
<br />
Qualification:<asp:TextBox ID="txtQualification" runat="server"></asp:TextBox>
<asp:LinkButton ID="lnkAdd" Text="Add More Qualification" runat="server" OnClick="AddMore"></asp:LinkButton>
</div>
<asp:Panel ID="pnlAddQualification" runat="server" CssClass="modalPopup" Style="display: none;">
<table border="0" cellpadding="0" cellspacing="0">
<tr class="header">
<th colspan="2">
Add More
</th>
</tr>
<tr>
<td>
Qualification:
</td>
<td>
<asp:TextBox ID="txtAddMoreQualification" runat="server"></asp:TextBox>
</td>
</tr>
</table>
<div class="footer">
<asp:Button ID="btnSave" runat="server" Text="Save" CssClass="yes" />
<asp:Button ID="btnCancel" runat="server" Text="Close" CssClass="no" />
</div>
</asp:Panel>
<cc1:ModalPopupExtender ID="mpeAddMore" runat="server" TargetControlID="lnkAdd" PopupControlID="pnlAddQualification"
CancelControlID="btnCancel" BackgroundCssClass="modalBackground">
</cc1:ModalPopupExtender>
</form>
C# Code
protected void AddMore(object sender, EventArgs e)
{
this.mpeAddMore.Show();
}
For better understanding of ModalPopup got below link
http://aspsnippets.com/Articles/Using-ASPNet-AJAX-ModalPopupExtender-for-adding-and-editing-rows-in-GridView.aspx