How to get a 144 hour visa

Hi, I've been looking into how to get a 144 hour free transit and I can't find any information other than the airline has to apply with our information to boarder control in SH and give us a letter of confirmation, but Cathay Pacific say they don't do it. Another company said to arrive at the airport and show our itinerary and passport, however someone said that we may get turned back.
Do you know how exactly we get a 144 hour free transit please? I'm so confused.
Thanks