Hello @SifakaCoquerel, unfortunately, making a custom VTC paint job visible for others is not possible.
If you want your to make your custom paint job to work on TruckersMP, you would have to create mod with your paint job a "local mod", but unfortunately, only the user is able to see local mods.
The only way to make it visible for others is by using one of basic SCS Software's painthobs.
There also is a lot of tutorials showing how to make a custom paint job in ETS2/ATS (singleplayer).