Regarding 72 hour transit at Shanghai (Tokyo - Shanghai - London)

Hi there, my airline Peach Aviation apparently isn't the best when it comes to responding to customers emails. Do you need to get your airline to submit the 72 hour transit for you prior to landing in Shanghai or can it be done when I arrive at the airport (approx 5.30am)? I will have all the correct documents i.e. onward flight proof to London, passport etc with me and only intend to stay in Shanghai until the next morning (but slightly over 24 hours). Thanks.