Clear a parameter at selection-screen

Hi guys, haven't found the answer to this one yet:

at my selection screen, I have the field p_name(40), it's default value is "Insert your name here", what I need is:

when the user selects the field to input data, the field must be cleared to receive the name.

already tried "at selection-screen" but it didn't react to field selection (or did I anything wrong?)

thank you