Open (Show) ASP.Net AJAX ModalPopupExtender Modal Popup on DropDownList change (selection) in ASP.Net

Here I have created sample that will help you out.

HTML

<div>
    <cc1:ToolkitScriptManager runat="server">
    </cc1:ToolkitScriptManager>
    Select Name :<asp:DropDownList ID="ddlNames" runat="server" OnSelectedIndexChanged="SelectedEventChnged"
        AutoPostBack="true">
        <asp:ListItem Text="David" />
        <asp:ListItem Text="Jhon" />
    </asp:DropDownList>
    <asp:LinkButton ID="lnkFake" runat="server" Style="display: none" />
    <!-- ModalPopupExtender -->
    <cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="lnkFake"
        CancelControlID="btnClose" BackgroundCssClass="modalBackground">
    </cc1:ModalPopupExtender>
    <asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" Style="display: none">
        This is an ASP.Net AJAX ModalPopupExtender Example<br />
        Welcome :
        <asp:Label ID="lblName" runat="server" />
        <br />
        <asp:Button ID="btnClose" runat="server" Text="Close" />
    </asp:Panel>
</div>

Screenshot