How to know which Transponder 4 digit number to insert on FBY A32NX before flight

this is provided by ATC. But typically speaking:

1200 - VFR in Class E or G Airspace
2000 - IFR but no flight following (or Oceanic)
2200 IFR with flight following but no plan on file
7700 General Emergency (do not use on VATSIM)