Pretty sure you can't get the soap to go through
the pressure washer. BUT you can get a foam cannon/foam lance that attaches to the end. Granted, this doesn't eliminate the need to hand wash it, but it does help loosen the dirt a bit and reduce the chances of marring/swirling. I use a foam gun with my regular hose, and my usual process is foam it down at a more concentrated solution, let it soak for a few minutes, spray it off, and then foam it down at a lower solution and go to town with the two bucket wash method.
-Helps minimize the chance of marring/swirling
-It's a ton of fun and very cool
-Use a bit more car shampoo than usual