Since community here is soo damm helpfull... I've never done it, but my best guess would be that you have to copy one of the license plate designs from the files that should be found somewhere in the common directory and then change the name to the desired one and edit the skin itself. Then you sill have to add a .jbeam file that basically connects the texture with the license plate itself. I'll take anlook at it once I get back home.
For some reason it doesn't like certain names... I've basically changed the same exact names a couple of times and it has started working... For some reason the file from the wiki doesn't work, so I've downloaded a random mod that worked, copied the files and changed every single name and redone the .png file. BeamNg modding can be a bit weird at times...
