Here's some sample code that will give you an idea. This will put the text in the box, but when they click on it, the text will disappear.
Content visible to verified customers only.
This is just some quickie code...you'll have to play around with it to get it to work right...and I'm not sure exactly where it would go.
You also have to make sure that the "this.value" and "value" match exactly, otherwise it wont replace it with blank space when they click on the box. And, if course, you'll have to make the NAME and SIZE variables match that of what's currently there.