Need a "server-side" solution.
Is there any way to access a control (Hidden HTML Field) from an IFRAME (RadWindow) that is on the parent page? Before I launch the IFRAME (RadWindow), I write the URL I am opening to a hidden HTML field on the parent page. The point is so I can access that value from within the IFRAME if I wanted to "go back" to the initial page that was opened. How can I access the hidden HTML control from within the IFRAME "server-side". I need to do this in the code behind.
How are you opening the RadWindow, after a postback? If you are opening the RadWindow using clientside unless you pass your hidden URL back to the server using Ajax or something then you will not be able to read it in the IFrame without then using Clientside script inside the RadWindow to read the parent value and then you would have to use a Cache or Session variable to be able to read it again from the Child window.
Depending on your situation can I suggest you pass the url to the RadWindow as a querystring value? eg
oWnd.open("window.aspx?backUrl=~/blah.aspx");
But you will probably want to UrlEncode the url value.