Assuming the scenery has a GSX L2 jetway that is configured with a Power Unit. In that case, connecting the jetway will automatically result in external power being available in the airplane.
For a realistic ground handling experience, FSLabs recommends using the add-on “Ground Services X” by FSDreamteam. GSX will provide the user with automated door, payload and fuel handling, as well as handling external connections to the aircraft. Furthermore, handling of the GSX software is fully incorporated within the A320X, doing away with the need of manually opening GSX menus. GSX will instead be triggered by doing the standard operating flows during flight preparations.
You have to use the GSX menu to dock the gate in the first instance, thereon FSL fully integrates with GSX.
How about Confirm good engine start? Do I need to open GSX menu or it should pop up on its own?