If you would like to have watermark in your textbox in asp.net to guide user to understand the functionality of the textbox than you can achieve that using JQuery with few lines of code.
In the below example I have used css and JQuery to create watermark effect in the textbox. In this example when user place the cursor than text will be removed from the text box and the css class will be normal, Again whithout entering if user leave the textbox than again watermark will be assign back in the textbox.
In the below example I have taken a asp.net textbox
and here is the JQuery code to craete watermark
Reset Button using JQuery in ASP.Net