Click "My Profile" at the top, then "About Me" in the ~middle of the page, and add it to your signature
At the top right of this page (on a laptop/Mac/PC) click on Settings. When the new page loads look middle/lower left column under Settings and click on Edit Profile. If you see a member's signature in a format you like, highlight & hit CTRL + C to copy. Then when you get to the Edit Profile section (see above) you can just hit CTRL + V to paste. From there just edit the text to your car's configuration. Not sure how to do that on device in Mobile Site view other than to click on Full Site (bottom right.)
Apparently you need to have logged some minimum number of posts before you can add a signature, probably 11.
Click HERE to add a signature. Alternatively, like BoerumHill described 3 messages above, on top right click "Settings", then on the left click "Signature".
Thank you everyone for helping me with this. It is fun to see everyone's car choice, age, options. I especially like reading posts from the early adopters.