To make sure you are set for this, here is what you need to do: Make sure you have updated your iPhone to iOS 18.2, open Find My app, find your missing item listed and tap on it, then tap to 'Share Item Location'. This will give you a webpage that you can share the link for with the airline.
The agreement, called Blue Sky, allows members of United's MileagePlus and JetBlue's TrueBlue to earn and redeem miles on most flights operated by the other airline.