Transponders can be recorded against your profile, and selected from a dropdown list when registering for an event. You'll see all transponders for the vehicles you own or borrow.
Adding a Transponder
Go to the Vehicles tab and click Transponders. On the pop-up screen, click Add New. Select the type, enter the transponder identifier, and select the vehicle this transponder is linked to.
Removing a Transponder
Go to the Vehicles tab and click Transponders. On the pop-up screen, select the transponder you want to remove, then click Remove.