Hi @erkamtb,
ArduSub currently only provides explicit joystick button function control over servos at outputs 9, 10, and 11 (via the servo_1/2/3_* functions), which is a carry-over from when the Pixhawk was our only supported flight controller board.
The other options are to use one of the alternative sets of joystick functions (e.g. for controlling lights, or camera tilt/pan), or to more directly use SET_SERVO MAVLink commands (e.g. via QGroundControl’s custom MAVLink functions, or programatically).