You just need to go to your profile and go to Forum Profile Information. Afterwards, you'll see an empty box in there that says
Signature. For Jr. Members, links are allowed but the characters are limited to just 150 characters.
Here is a list of ranks and their allowed signature styles:
- Newbie: No styling (including links) allowed. Max 50 characters.
- Jr. Member: Links allowed. Max 150 characters.
- Member: Unlimited length.
- Full: Color allowed.
- Sr. Member: Size allowed
- Hero: Background color allowed