You can't. You're a newbie.
Due to excessive garbage-posting, you now need 1 merit to be a Jr Member. All existing Jr Members who didn't meet the requirement were demoted. Also, newbies can no longer set
any signature or personal text.
Many of these limitations can be eliminated with a
copper membership. In particular, you can set a signature at Member level if you wear a copper membership.
So you're going to have to either earn 1 merit for your posts or buy a copper membership.