The Agent key code is the same as your RE/MAX Mainstreet Username. Your Mainstreet Username is the portion of your remax.net email address that appears before the @ symbol. (i.e. email@example.com johndoe is the Username.)
You can also view your Agent key code by logging into Launchpad > Mainstreet > select the 'Marketing' drop down box > choose 'Agent Branding for Mobile'.