Hello,

it would be good to add the option to start and shut down notifications to a specific channel.

All notification messages go to the first defined channel which doesn't necessarily have to be your main chat channel

So how do I set up a different channel for power-on and power-off notifications?

Server start, stop, player advancement/achievement, player join, and player death all go to the same channel. Distinct channels per event isn't something that's implemented.

And how do I choose that channel?

It's the first channel pair that you have in your Channels option.