Hi Triet Nguyen Cong,
The RadDatePicker does not support watermark.
However it can be achieved with some editing of the ControlTemplate of the RadDatePicker and some additional code. I created an Watermark class that has Text attached property. Also I put a TextBlock in the RadDatePicker that has Text bound to that property. The Visibility of that TextBlock is bound with converter to the SelectedDate of the datepicker and it's opacity is set to 0 when the datepicker is focused. I marked the template changes with "" comments check them as well as the code behind for the example. You can reuse the code form the example in your project.
I hope it works well enough for you.
the Telerik team
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