Sys.Net.WebRequest

I'm trying do do a HTTP Post to fetch the html resulf of an .ascx page.  Joe Stagner said this was possible in his video entitled "Implement the Incremental Page Display Pattern using HTTP GET and POST".  He said we could fetch .htm, .aspx or .ascx pages, but with i specity a .ascx page in the url property of the WebRequest object I get the message below.  Any help would be greatly appreciated.  I include my code snippet too.
 
<script language="javascript" type="text/javascript">  
function PostWebRequest(post ...