You could modify the CaretBrush by setting the TextBoxStyle property of the AutoCompleteBox to a custom Style which is BasedOn the default AutoCompleteTextBoxStyle. Afterwards in the custom Style you just need to set the desired properties of the WatermarkTextBox. I have prepared a sample project which demonstrates the exact approach, note that I have used Implicit Styles in order to be able to base on the default style.
Hope this works for you.
Regards,
Kalin
Telerik
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