Add message under submited post?

[ZomgStuff]

Could someone please help me with this in php. I have faint knowledge on php although I am reading up on it. How can I add a auto message to the end of someone's post that they're submitting? My goal is to later add a textbox for them to input a link, and it will wrap some preset bbcode around it to format it the way I need.

Thanks!

[FractalizeR]

Look at Custom Thread Fields mod.