For future viewers of this thread, the API key is the same as the MAC address. You can usually find this on your router config where it shows device info. The MAC is also shown in the serial output when you connect a burner device (or FTDI cable as mentioned above.
My problem was the device was stuck and needed reflashing. Once I did that it worked perfectly using the 12-digit MAC as the API key.
You may safely ignore the 8 digit "API Key" that is shown in M5burner or the command line, this was a red herring.