Spining BLDC motor in cooling fan during arming

Currently I’m building gas hali with Pixhawk2.
In this working, fan belt is broken easily due to engine heating, so I’m trying to replace with fan spined by BLDC motor on arming state.

So I would like to know how to generate PWM signal to any servo port for BLDC motor working.

Is there any servo function via Mission Planner setting?

If not, I have to try it by Arducopter code change.
I’ve found the funtion to check arming, motors->armed().
But I don’t know how can I write proper PWM to servor output.
Anybody can help me? Thanks.