meshtastic --set mqtt.address "<own mqtt>" --set mqtt.username "<user>" --set 
mqtt.password "<password>"

Connected to radio
Set mqtt.address to <own mqtt>
Set mqtt.username to <user>
Set mqtt.password to <password>
Writing modified preferences to device
Writing mqtt configuration to device

meshtastic --get mqtt.address
Connected to radio
mqtt.address: mqtt.meshtastic.org
Completed getting preferences