I entered "0000" for the pin. I think the best option is to complete all of the fields, relevant or not. Another odd field is the passport field. The USA address doesn't translate well internationally, but better to complete all of the fields. The booking engine isn't very smart. Calling customer service after the fact may or may not be of value; but I tend to think calling customer service with LifeMiles isn't of value.

I booked round trip ord - lhr on United and was very pleased to complete check-in. I learned long ago to go to the airport with passport in hand and complete check-in in 24 hours in advance. In the event of issue the staff at the airport may be of assistance in light of the 24 hours in advance. The staff at the airport have no interest in sorting out a broken international ticket 1-2 hours prior to departure. The rule of thumb is to call LifeMiles but with sufficient time the first airline on the itinerary may be of assistance (and contact LifeMiles directly).

The account profile does have an option for a pin. I have a four digit pin noted in the details of the LifeMiles account but I have no recollection of the source of the pin. I think you can go online and set the pin in advance of calling customer service to expedite the call to customer service. Regardless, no indication the two "pins" are related...