This article explains how to set the Watermark of RadTextBox.
RadTextBox provides the ability to set a watermark. This is a text, which is displayed before the input has started and can be used to give the end user with an idea about what should be filled in this text box. When the text box is on focus the Watermark disappears and when the focus is lost it appears again if the field is empty.
Here is an example:
<telerikPrimitives:RadTextBox ActionButtonVisibility="Visible" Watermark="search"/>
And here is the snapshot of the result: