Actually, you don't have to use the DisplayMask property in this case, it is intended for other purposes.
I think you need:
<telerik:RadMaskedTextBox PromptChar=" " DisplayPromptChar=" "
If you set the DisplayMask to a sequence of spaces, you will not see the entered value when the textbox is not focused.
Let me know if I am missing something.
the Telerik team
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