As far as i know, you can't hack a certain car(s). you can only obtain them by purchasing them or winning them on their own event, other method will get you ban.
The script provided by @Enyby works all the time for me - but my personal advise is not to run the script more than once, if you need to (re)run another event script (e.g. you have two events currently active) you need to restart the game first to reset the previous alteration.
Also, all server-sided stuff (tokens, BP, permanent pro upgrade, licenses) can't be alter thru our GameGuardian app, even if you did this will not sync on your game data the next time you connect the game online.
You can alter the Pro upgrade of the car temporarily but it can only run on their own event (R&D, EDD, championship or World Tour), never run them on TLE and MP - this will put you to cheaterboard (CB) and eventually, ban.
At least we can do the trick for Free class upgrades, credits, ET and Max upgrade.
Lastly, never ever make an alteration to the Mastery rewards. This will surely ban you when you claim it. I lost many account due to this.
This is all based from my experience, please correct me @Enyby if there are part that I'm wrong.