Can't find anything after 2018 on this, but i used to be to just spawn in a couple vehicles and then save the map in the editor and it would save the vehicles so i don't have to respawn them but now it doesn't seem to work. Made sure "save enabled" was checked too.
Oh i did another method which i figured out here:https://www.beamng.com/threads/can-i-save-a-map-state-somehow.65770/. I basically made a folder in the scene treee for the vehicles and then mase sure save enabled was on. I think it's something to do with how they changed spawned vehicles in freeroam to their own folder in the scebe tree.