Displaying a .aspx page in a pop up window on buttn click event

First of all, please kiss away the thought of using any of the default or built-in pop up windows for doing this, such as alert dialog boxes etc.

If you want to show a pop-up window, you create one using a simple HTML container element, which is a simple div element. Now, the code to load that page is pretty much simple, you would use a simple Ajax request to request for the data. You're not concerned with any of the C# code here, you just want to render the HTML document here. So, create a HTML web-page as this one (I am going to write the code for pop-up window only),

<div class="pop-up" style="display: none;">
   <!-- Leave it empty -->
</div>

.. you can add other styles to make it float over other elements too. Now, write the JavaScript code to create a new Ajax request and call for data. Like this,

// I will use jQuery here..
$(document).ready(function () {
   // create the request here, 
   $.ajax({
      url: 'link-of-page-to-download',
      success: function (data) {
         /* upon success, 
          * chances are that entire HTML will be passed down, 
          * do not worry about any return statements, the document will be sent
          * down into the data variable here.. 
          */
         $('.pop-up').html(data);
         // Show the pop up, 
         $('.pop-up').show();
      }
   });
});

In the above code, if there is no error in the ajax request, the resources (HTML document and other data attached; required, to it) will be downloaded to your page and then they will be added to the pop-up window which at that time is empty. Once that is done, it will be shown using that method .show().

For more information on jQuery ajax, please read this document[^].