The desired scenario can be easily implemented by modifying the default Prompt ControlTemplate in order to disable the Ok Button if the TextBox is empty using ValueConverter. And you can also remove the Cancel Button and the Close Button, so the user will be only able to press Ok if there is entered text. I have prepared a sample project which demonstrates the exact approach, note that I have used Implicit Styles to Style the Window.
Hope this helps.
Regards,
Kalin
Telerik
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