Second, a complaint is not even a legal term. That just a general term to describ you are not satisfied for the serves you received. It is nothing to do with law. After you file a lawsuit, a complaint just part of your proof that you were looking for solving problems. You are comfused.
Third, if you really need to send them a legal document that don't know where to go, call them, or if your English not good enough, email them. They will tell you.