If you are using Tapatalk, click 'Edit' then 'Edit Title' and tick one of the boxes for the required prefix.
If you are using the web forum there should be a drop down selector to the left of the Title. You can choose a prefix there.
It was five mintues initially then I set it to unlimited and now settled on a 60 minute limit. If something needs editing after the limit you can report the post and one of the mods can sort it for you.