I have acquired a green hat recently.
Does anyone have an idea as to how I make it show up on forums
Like when people have the green hats on their names and such.

iirc properly, it's actually just a normal ingame 3D item

Answered in irc, closing

For those that might want the answer as well: The green hat item you can buy is not the same as the usertitle. Usertitles were given away to those that purchased it or won it in some competition (in my case), when they were available. They are no longer available.