I want to install an overhead microwave?

The best thing to do is follow the manufactuers instructions. However you might also find this answer helpful, someone ask the same question about a week ago, here is the link:

https://www.askmehelpdesk.com/appliances/how-do-install-overhead-microwave-oven-39857.html

One thing to check before you buy one is if this will take the place of a hood fan, many times the fan is not wired on a 20 amp appliance circuit , but on on the kitchen lighting circuit.

A microwave can use anywhere from 9 - 12 amps, and this may cause the lights to blink, and may be an overload to the circuit with the existing load.