You could use some of the approaches in the demos provided in my previous reply to validate the web address. First, choose what kind of control you would like to use, whether the Telerik RadTextBox or the default asp:TextBox. Note that you could not use Telerik RadTextBox with the RadInputManager like you tried in your first post.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed