You can achieve the described requirement with the following approach:
<Style TargetType="telerik:RadDateTimePicker" >
<Setter Property="DateTimeWatermarkContent" Value="Test" />
<Trigger Property="IsEnabled" Value="False">
<Setter Property="DateTimeWatermarkContent" Value="" />
Note, that if you are using the NoXaml dlls, you have to base the style on the default one for the theme.
I hope you find this helpful.
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