The pound character "#" is part of the standard masks
, so you will have to set the MaskType to Standard in order to use it:
RadMaskedEditBox1.MaskType = Telerik.WinControls.UI.MaskType.Standard
Please note that each pound sign represents a single digit, so if you want to use this approach, without an initial value, you need to add as many pound signs as the maximum number of allowed digits in the control.
I hope this helps.
All the best,
the Telerik team