There is still an exploit that gives you 7500 xp every ~30 seconds or so, you can't do it AFK and it honestly doesn't really get you THAT much more than if you just played and raced regularly I feel. It still uses the RAM car and the test track so I think just doing that also works, this just shortcuts it so you don't have to complete lap or anything. You have to follow these directions to the step, doing a 30 second timed first then the drift bucket change.
https://www.youtube.com/watch?v=e3V_OwEjDac
I did it for about 15 mins, got ~100k credits, and got so bored I just couldn't do it anymore. I'm not sure why you'd keep doing it to rack up credits, if you just play you'll get plenty and a few weeks of forza rewards will have you drowning in money too.
Still, I feel in FH3 I was getting a lot more credits just doing stuff that was fun for me whereas here I feel very constrained.