From the code snippet you sent I take it that the URL page you want to add to the RadTabItem.Content is basically a UserControl. Is that the case?
If it is, you don't need to use a URL to add it to the RadTabItem. You can add it like so:
RadTabItem itemToAdd = newRadTabItem()
Header = _Header
itemToAdd.Content = newThatPage();
You can fine more info about how to add UserControls to your main page here. I hope this will help you.
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