how to call a page as popup on button click

You can use javascript to open popup window.it can be a HTML or ASPX page.
In button client click event call the javascript to open the popup window.
here is the Sample code.

<script type="text/javascript">

    function openPopup() {

            window.open("Default2.aspx", "_blank", "WIDTH=1080,HEIGHT=790,scrollbars=no, menubar=no,resizable=yes,directories=no,location=no");  
               
                }

</script>

  <asp:imagebutton id="btnpopup" runat="server" onclientclick="return openPopup()" />

If you want to call the Javascript function during your Server Side button click event

  protected void Page_Init(object source, System.EventArgs e)
    {
  btnpopup.Attributes.Add("onclick", "return openPopup();");
}