When you register a trademark, you do it through the United States Patent and Trademark Office (USPTO). Once you have a trademark license agreement, you should record it with the Assignment Services Division of the USPTO. Be careful doing so because this becomes public domain, and some free trademark license agreements might include commercially sensitive or confidential information or trade secrets you don't necessarily want to be publicized. You can redact some sections of your license agreement or file a separate document that contains all of the pertinent information without disclosing sensitive information.