You can get a reference to the iframe in the RadWindow by using the control's Client-side API
oWnd = radopen(url, wndName);
Note that the popup must be shown already in order to have its iframe created. Thus, the earliest point where the if block can be added is the OnClientShow event. The event handler can be set for the manager if you want it for all RadWindows. The event handler would receive the RadWindow instance as its first parameter, so you can use that.
All the best,
the Telerik team
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