You'll want to use "Forum Code #1" and you'll want to use it as your Signature Text, not as your Signature Graphic/Image.
[1] Go to "User CP"
[2] Click on "Edit Signature"
[3] In the text editor window labeled "Edit Signature" paste the text from the registry. Do NOT use the "Signature Picture" section.
Then you should be all set

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Register your Dodge Challenger at the Unofficial Dodge Challenger Registry!
Performance Mods: Mopar CAI, Kooks Long Tube Headers, Corsa Exhaust, BT Catch Can, Jet 180 TStat, Hurst Short Shifter, Predator (CMR Tune [Livernois Motorsports])
Cosmetic Mods: 35% Tint, BT Underhood Kit, Door Sills, WebElectric Sequential Lights