You will need to look on the device to discover the device MAC address. If it's a half-decent device it should be on a sticker somewhere. Worst case you might need to pair it with a phone or other desktop computer and then look at the connection details to find it out before unpairing and then using the info to pair in LE.
Thank you. I was thinking about that and wondering what to pair it with, but then one day the GUI bluetooth menu decided to work for some reason. When I turned the keyboard on the menu showed the manufacturer and device, and I was able to trust/connect/pair eventually by doing it in the right sequence. I noted the MAC address now in case I have to re-pair it, I can try the SSH/bluetoothctl method.