Open the boarding pass in your Apple Wallet; in the upper right corner, you will see a button "…". Just tap it.
Select "Share Pass" in the upper right corner.
Select App in the Air. If App in the Air is not on the list, tap More and select an app.
The boarding pass will appear as a card in the list of your flights, on the "Boarding Pass" widget:
If you see an error when adding your flight → share this Wallet file at firstname.lastname@example.org. We'll review it.