Isn't that technically editing the thread title? Only moderators can edit thread titles.
More a assumption than anything else, so wait for confirmation. I can't find any way to add/remove a prefix as well (just tried it).
In case it is a (moderation-) privilege thing, you could report your own thread and nicely request the change.
If you click edit on your first post you see no options to add a prefix?
From a moderation point of view changing prefixes at least seems to be available: