Dear @TonyS,
Deep Links could be used as a generic term:
For Mobile apps they provide a way to open a specific app page within the app from external source.
For web apps they represent a URL inside the app where the user should be redirected.
Your question was how to define a specific URL in Adalo PWA, which you would like to use in your payment API call. I have mentioned several possible options in my answer above.
Also, if you do a search in the forum using keyword “deeplinks”, the 2nd result is a link to the tutorial about deeplinks PWA custom component: PWA Deep Linking
Victor