Under vehicle power wash

I have one. It works well, but I can't figure how to apply the soap. My regular pressure washer wand has a low pressure setting that siphons soap out of a tank. On the high pressure setting, no soap, but pressure. The undercarriage attachment only has a high pressure setting.

To clean my vehicles undercarriage, I sprayed soap on my undercarriage from a spray bottle, let it soak, and then washed off. It works, but not as convenient as I hoped.
 
That's funny, I just bought one like this, but the nozzles aim down to act like a broom, and I was thinking how to modify it to spray up. It does work great for cleaning the driveway after washing all the mud off.
 
I have one too. Works well. I’ve not used it with soap, just plain water to wash the underside off after beach trips.