Hi ramco1917,
You need to add AjaxControlToolkit library.
Under the Panel you need to add the IFrame that will be used to open the Web Form in the Panel.
Please refer below sample.
HTML
Default
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<style type="text/css">
.Background { background-color: Black; filter: alpha(opacity=90); opacity: 0.8; }
.Popup { background-color: #FFFFFF; border-width: 3px; border-style: solid; border-color: black; padding-top: 10px; padding-left: 10px; width: 400px; height: 350px; }
.lbl { font-size: 16px; font-style: italic; font-weight: bold; }
</style>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:Button ID="Button1" runat="server" Text="Show Popup" />
<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panl1" TargetControlID="Button1"
CancelControlID="btnClose" BackgroundCssClass="Background">
</cc1:ModalPopupExtender>
<asp:Panel ID="Panl1" runat="server" CssClass="Popup" align="center" Style="display: none">
<iframe style="width: 350px; height: 300px;" id="irm1" src="Contact.aspx" runat="server">
</iframe>
<br />
<asp:Button ID="btnClose" runat="server" Text="Close" />
</asp:Panel>
Contact
<table>
<tr>
<td>
<asp:Label ID="lblName" runat="server" CssClass="lbl" Text="Name"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtName" runat="server" Font-Size="14px"></asp:TextBox>
</td>
</tr>
<tr>
<td>
<asp:Label ID="lblCity" runat="server" CssClass="lbl" Text="City"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtCity" runat="server" Font-Size="14px"></asp:TextBox>
</td>
</tr>
<tr>
<td>
<asp:Label ID="lblState" runat="server" CssClass="lbl" Text="State"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtState" runat="server" Font-Size="14px"></asp:TextBox>
</td>
</tr>
</table>
Screenshot
