I would say, you need to be a minimum member rank to be able to have a link in your signature. Check this topic and find all rules and limitations about member ranks:

Forum ranks/positions/badges (What do those shiny coins under my name mean?)

and what you are particularly interested in:

You're not allowed to put links as a signature. You need to rank up to the member rank for that, which requires 10 merits.

For more information: https://bitcointalk.org/index.php?topic=178608.0

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On the other hand, if you really really want to advertise on your signature asap, you can purchase the Copper Member rank: https://bitcointalk.org/index.php?action=credit;promote