How to stop fixed-wing propeller spinning after arming?

Hello Everyone

I am trying to use px4 for a custom airframe which launches with catapult. I configured the mixers but there is a problem which is after arming the autopilot, motor starts to spin, but this will cause a problem in launching catapult.
I configured the Catapult parameters and I have set the PWM_MIN=800.

In my case, launcher shoots the plane with a very high acceleration and after about 1 second from the sensing of the acceleration motors should start spinning.

how can i do this?

Your help would be much appreciated.