you can either open the tailgate (in most vehicles) and drag in another spawned object, or you can use the parts selector (in only some cars) to give it a roof rack and load things in there in the selector. I think the trailer is the only vehicle that can have predefined loads
You can use my mod (https://www.beamng.com/resources/un...oads-read-description-for-instructions.25824/) that adds, among other things, an option for trunk loads for base game vehicles. It might be a bit broken currently for a few cars, but for the most part works well.