You indeed require 500 posts to request a custom text usertitle, image usertitles require 1,337 posts.
If you had atleast 1,000 posts you could've requested it here: Staff Requests - MPGH - MultiPlayer Game Hacking
But since you don't have 1,000 posts yet, you can just request it within the Suggestions, Requests and Help section: edit your post with the desired usertitle.