Good day. Thank you for your question.
May I know if you are looking for the wiring diagram to connect a buzzer? If yes, kindly refer to page 5 in this link https://teltonika-gps.com/downloads/en/fmm125/FMM125-Quick-Manual-v1.7.pdf
Regarding the command, you may refer here - https://wiki.teltonika-gps.com/view/FMB_setdigout Since FMM125 has 1 DOUT so you may refer to the part where it indicates "For devices with 1 digital output"
Hope this helps.