You need to tick the option "Require users to select a prefix" in the Thread Prefixes tab in a node.
Also note that it says:
If selected, users will be required to select a prefix when creating a thread or updating its title. This will not be enforced for moderators or when moving a thread.
at least when it comes to creating threads. I've just tried and I have to select one, even though I am a moderator (in the ACP with full permissions). @Mike@Chris D has this changed? Or maybe it's a typo and should read:
This will not be enforced for moderators when moving a thread.
Thanks for quick answer again! I love xenforo community and its support forums!
I am a total newbie and i just have to say thank you to all the "vets" that are using there freetime to help out ppl like me!