How to apply custom license plate to secondary characters vehicle?

[bronze cairn]

As the title says, how can this be done to the secondary character as I can't change the license plate

[sage garnet]

Secondary character, as in other cars that aren’t on the list of your most recent personal vehicles? If so, all you need to do is add your chosen plate on one of them (and I just change it back immediately) but afterwards the new tag should be in the system and you can then add it onto any vehicle.

For instance, Lamar used to have a Vapid Speedo.