A little late but I researched this as I am buying a Rocket 3 GT shortly so thought I'd give a little more clarity to those that come after me,
This is explained in the manual, p64. To turn the motorcycle on with the standard key (or the smart key if the battery is discharged):
• Hold the standard key on the system sensor located on the left hand side of the motorcycle.
• The standard key must be held against the system sensor while pressing the Engine Start/Stop switch in either the QUICK START or Power ON/OFF position.