You'll probably be told this is the completely wrong place to ask for this but until they get to you here's how:
Click on "edit profile" next to your profile pic near the top of the page, and if you scroll down to the bottom of the first page you come to you'll find a box with the title "Set Message Boards Comments Signature:". Just put whatever you want in there and then click "Submit" right at the bottom".
Thanks, must have been blind.
Kris