Arduino and firmata unique key

Hi
I was wondering how to allocate or find the unique key for my arduino running firmata ?

I have seen code like

firmata:
  - serial_port: /dev/serial/by-id/**usb-Teensyduino_USB_Serial_358320-if00**
    serial_baud_rate: 57600
    switches:

I used a direct reference to my serial port ttyUSB0 but on rebooting it cannot find it I’m guessing the port was reallocated as I had pluged in some other serial/usb devices.
Thank you
Mark