Can I add my customer's frequent flyer number to a booking made through a flight booking API?

Yes, in most cases you can. The loyalty number is sent to the airline together with the passenger details at the time of booking, so it sits on the ticket from the start and your customer does not have to go back and add it later.

Whether it actually works depends on the airline. Full service airlines that belong to a large alliance usually accept a number from a partner airline as well as their own, so a customer can often earn miles on a flight operated by another member of the same group. Many low cost carriers have no programme at all, or accept the number only on their own website. The booking request will normally carry the field either way, but it is the airline that decides whether it is honoured.

It is also worth being honest with your customer that adding the number does not guarantee the miles. Airlines award miles according to the fare that was bought, and the cheapest fares sometimes earn very little or nothing. The number also has to match the passenger name as it is held in the loyalty account, otherwise the airline will simply ignore it without telling anyone.

Before you promise anything, check with your supplier or your account team which airlines in your content accept a loyalty number through the booking flow, and tell your customer to keep the booking reference so they can claim any missing miles directly with the airline afterwards.