Actuator or Servo Control Mid Mission

I am trying to control a servo or actuator mid mission. I’m running PX4 v1.14 with QGC and Flightgear as SITL, with a fixedwing aircraft heavily based upon the Rascal. I have tried creating mission items using QGroundControl to set a particular servo to a particular PWM (MAV_CMD_DO_SET_SERVO #183), however monitoring that servo during the mission shows no change to the PWM at all. Is there a trick to this I’m missing?

Can anyone tell me if such a function is supported in PX4 fixed-wing?

Thanks very much for your help in advance!

Kind regards,