72 Hour Visa Free Transit. Does anyone have any feedback or experience on the process?

Hello All - I'm flying United from Los Angeles (LAX) to Shanghai (PVG) arriving Thursday March 20th and departing Sunday March 23rd to go to Cambodia. All information I read says I am eliglble to do the 72 hour free transit but I must register with the carrying airline first. I can't seem to get any additional information from United and don't want to not be able to board my flight in Los Angeles.

I have a confirmed/booked ticket for both the Thursday incoming and Sunday outgoing flight.