you have do
1.Add reference of user control on page.
2.Add ToolkitScriptManager on page.
3.Your model popup
4.In panel add usercontrol
and do designing something like as follow
<asp:ToolkitScriptManager ID="ScriptManager1" runat="server">
</asp:ToolkitScriptManager>
<asp:Button runat="server" ID="hidden1" style="display:none"/>
<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server"
PopupControlID="Panel1" TargetControlID="hidden1">
</asp:ModalPopupExtender>
<asp:Panel ID="Panel1" runat="server" BackColor="Aqua">
/*here add ur user control*/
<br />
</asp:Panel>
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