I must be blind?
 

How do I set up a forum so that there is an option to add a prefix to each post that can be selected? Is this a hack and not a baseline option?

there is Thread Prefixes in admincp to add thread prefixes

Admin CP -> Thread Prefixes -> Add New Prefix Set

Once added, it'll allow you to select forums to use that prefix (hold shift to select consecutive options or hold ctrl and click related forums). If you want to add any more, go to:
Admin CP -> Forum Manager -> {Forum} -> Edit Forum -> Use Selected Prefix Sets -> Tick relative boxes