I wish I could tell you, but I can add the Prefix "For Sale" or "Sold" to anyone's thread in the Classified area by Editing Thread from a Thread Tools pull down at the top of the thread, which is probably a Moderator function, which isn't going to be much use to you!
Worth looking though. On Blade Forums, where I am no longer a paying member, but was, I could go to Thread Tools at the start of a thread that I created, pull down and Edit Title, which brought up a window in which I could select one of the pre-set prefixes. Can you do that here?
Chris