Control servo pwm output via MAVLink command

I am creating the vehicle which will be transporting three or four payloads which will be deployed on my command sent by app. I created app for this task in C# to control the vehicle and plan the mission. Below I posted my questions.

  1. What command should I use to set open or close pwm signal to the specific servo?
    I found out that there is two command DO_SET_SERVO and DO_SET_ACTUATOR. The first one is unsupported by the autopilot I use (Holybro Pixhawk 4). The second one requested from me to indicate which servo should have specific pwm.
    MAV_CMD_DO_SET_ACTUATOR

And there goes another question.

  1. How do I know which servo is which? Could someone confirm if my understand is correct?
    Actuator 1 is MAIN1, Actuator 2 is MAIN 2, …, Actuator 6 is MAIN 6 (index is 0)
    Then I increase index to 1 and:
    Actuator 1 is MAIN7, Actuator 2 is MAIN8, Actuator 3 is AUX1 …
    Is it correct?

It is needed for me to deploy payload during misision flight.