mavros : Which waypoint parameter needs to be set to operate servo in mission mode?

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Hi I'm still not good at px4 and mavros though I started to play with pixhawk and Ubuntu...

That's a pretty long question title, but I really need answer(or someone's help maybe)

In Ubuntu and PX4 relation, when I need to make waypoint mission in ubuntu,

I can set waypoint array and modify each array's parameters, and push back to PX4.

Question is, which parameter should be set to operate servo motor in mission mode?

I know there's a lot of documents about that, but what all the documents say are different.

Any reply helps me. Thanks!