Open the boarding pass in your Apple Wallet; in the bottom right-hand corner you will see the “...” button. Just tap it.
Choose “Share Pass” on the popped-up screen.
Select App in the Air. If App in the Air is not on the list, tap “More” and select the app.
The boarding pass will appear as a card on your flight list, at "Boarding pass" widget":
If you saw an error while adding your flight → share this Wallet file to firstname.lastname@example.org. We will take a look at it.