How do I enable certain prefixes in my board.

Example 
Section : Market Place.
PREFIXES : [WTB] [WTS]

In the settings for your forum boards, you can enable Tags and Prefixes

 

Pity that there is almost no difference between tags and prefixes in IPS.

Would be nice if there was an option to create per forum limited configurable prefixes.

At this point tags/prefixes are either open or closed. Would be nice if this could be a per forum setting so you could choose per forum some prefixes where the user has to choose from and maybe leave them free to use tags for postings in other forums.

As far as I know this is not the case. If that is correct, it might put in a feature request for this. But maybe this has been spoken about some time earlier, I don't know.

There might be an app in the Marketplace for advanced tags and Prefixes.