You are talking of
Textbox Watermarks. Like the one we have here in CP for Search in the Top Green bar just below the Ad at top-right corner.
It is done via using Javascript. You have few options:
1. Write your own Javasscript for the similar behavior on the fly as and when required - You would need to raise an event onfocus/onclick/onkeydown (any such event) of the textbox and clear it's value.
2. Make a custom control that has this behaviour and use it.
3. Use existing thrid party controls. For example:
AJAX toolkit TextBoxWatermark Demonstration[
^]
I would prefer 2 or 3.
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