What I suggest is:
- Put a RadAjaxLoadingPanel on the parent page.
- When the button in your first page is clicked (the one that will redirect to the second page), show the RadAjaxLoadingPanel. You could use its API to position it over the RadWindow so it will look like it is in the content page, but will actually stay on the parent. This will allow you to show loading animation while the page in the RadWindow's IFRAME is redirected.
The information about explicitly showing RadAjaxLoadinPanel is available in the help article that I linked in my previous reply. This help
article will show you how to call a function on the parent page from within RadWindow.
the Telerik team