You can use any of the three: special services, gate, or customer service desk after security. I do not risk the gate any more as sometimes the gate isn't staffed or has been changed very close to boarding. I've had my extra seat "auto checked-in" by the computer and then moved for whatever reason less than an hour prior to boarding. The customer service desk after security is usually a good bet unless you get unlucky and all of the employees are taken by folks who have very complex questions.