auto fill form line form popup window

well i will start off saying i know very little of JS.

i am trying to find something to learn how to auto fill a form line from a popup window. Kinda like how phpbb does it with there user search function. i tried to find something on it but havnt had much luck since i have no idea what to really look for hehe

I think the parent window is something like window.opener not sure though :)