The demo that you refer to (
) is the demo for RadWindow for ASP.NET while Princy posted a link from a help article about RadWindow for ASP.NET AJAX.
I am not sure what is the exact version of the control that you are using. In general, if you use RadWindow for ASP.NET (shown in your link), you should check this help article:
If you are using RadWindow for ASP.NET AJAX, I believe that the following KB article will be of help:
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