ModalPopupExtender from Server Side Code

<!-- Hidden Field -->
<asp:HiddenField ID="hidForModel" runat="server" />
 
<asp:ModalPopupExtender
ID="WarningModal"
TargetControlID="hidForModel"
runat="server"
CancelControlID="btnWarning"
DropShadow="true"
PopupControlID="pnlIssues" >
</asp:ModalPopupExtender>
 
<!-- Panel -->
<asp:Panel ID="pnlIssues" runat="server"  
BorderColor="Black" BorderStyle="Outset"  
BorderWidth="2" BackColor="Wheat" Width="400px"  Height="106px">
   <center>
       <h2 class="style2">
           Information</h2>
       <p>
 
         <h3> <asp:Label ID="lblWarning" 
runat="server"> </asp:Label></h3>
       </p>
 
 <!-- Label in the Panel to turn off the popup -->
 <asp:ImageButton ID="btnWarning" runat="server"
                ImageUrl="~/images/buttons/update.png" />
</center>
 
</asp:Panel>

WarningModal.Show();
lblWarning.Text = "This is a popup warning";