This feature is not available for RadDatePicker for WPF. Unfortunately with the current implementation of the control no workaround can be made as well. Usually this can be achieved by setting IsDropDownOpen= true
event, however in this case when the dropdown which is a popup opens it takes the focus and when close it, the focus goes back to the RadDatePicker and the popup is opened again so you go to an infinite loop.
I have logged this as a feature request in our PITS, you can follow its progress and vote for it here
the Telerik team
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