Are you in the 'For Sale by Owner' forum/section when you hit the '+ POST NEW THREAD' button? :dontknow:
The prefixes are forum/section dependent, and some forum/sections don't get any prefix options; so if you aren't in the correct forum/section area when you try to start your new advert thread, you may not get to see the little Prefix box above the Title field... In the 'For Sale by Owner' forum/section, when you first see the empty 'New Thread' screen, the Prefix box should be up the top right and it should show
(no prefix) with a black 'down arrow' to its right - hit the down arrow to open up the selection box, then you might need to scroll down a touch to see the FS: option so you can select it. Hope this helps.
Good Luck! :thumbup: