Hello Julien,
The OnClientBeforeClose event is handled in the main page, so this is where the confirmation logic must reside. In order to check the hidden filed you need to access the RadWindow's iframe first and call a function that will return the value of the hidden field. The approach for calling such a function is explained in this help article:
http://www.telerik.com/help/aspnet-ajax/window-programming-calling-functions.html.
Greetings,
Marin
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