jQuery. Any manipulation of the form after it has been rendered on the clients's browser must involve javascript and possibly AJAX for serverside stuff.
Here is a simliar idea
http://stackoverflow.com/questions/558445/dynamically-fill-in-form-values-with-jquery
and then use the the javascript onblur event (the action of clicking into the field) , to erase the value before the lead starts typing into it.
For details
http://www.mkyong.com/jquery/jquery-form-events-examples/
Look at the second example
http://javascript.info/tutorial/focus-blur
Hope this helps
Eric