change the value after posting

[DanielM]

hay,
i added new input to new thread called "username", which users can put there username of one of the forum members and tell him that this thread is about him.
i built all but i have one problem. if the user changes username so it wont saved. i want that after posting the thread, the value of the input username will changed to the uid of the member. how to do that? and where? i can do that but i dont know where to do that. which file and which line.