The Services board, you'll find it
here.
Start an announcement (ANN) thread and then give us all the great word about your service along with the details of the contest. You'll get plenty of attention, I'm sure of it!
I wouldn't set up a separate thread for the contest because you'll need traffic to the new ANN thread you create and a contest is a great way to do it. Just edit the original post once the contest has ended.
Good luck!