The Cook-Key® ID is on the back label of the Cook-Key® Device.
Furthermore, when the Cook-Key® is connected to the Thermomix® TM5, you may find the Cook-Key® ID and MAC address under Menu -> Settings -> Help -> About Thermomix®
Important: Please do not disconnect the device from the Thermomix® TM5 without first eject the Cook-Key® from the setting menu on your Thermomix® TM5. Menu -> Settings -> Cook-Key® eject -> Confirm.