Windows Key + R > resmon.exe > right click “FlightSimulator.exe” and hit suspend process. Resume it the same way once you want it to run again.
Don’t get your hopes up though, the amount of energy this will save would be hard to see on a power bill, and it will do a hairs width away from literally nothing to help the climate. Even if every single flight sim user did this whenever they stepped away from the PC, it would barely change anything in the grand scheme of things.