Originally Posted by anabolism

I book codeshares all the time and rarely have problems with seat selection. I haven't tried an AS code on a JL-operated flight, but your TA as well as AS and JL should be able to allocate seats for you. With a codeshare, there will often be a different record locator for the operating carrier of that specific flight, so there might be a JL record locator for the overall itinerary plus a second JL-only record locator for the JL flight that's booked as an AS flight in the main itinerary. In this second record, the JL flight will be a JL prime flight, but it is a "limited services record," meaning that it can only be used for seat assignments, checking in, meal selection, and potentially an upgrade once under airport control. So first I'd try to have your TA allocate seats in their booking engine. If that fails, have them dig in and see if there's a separate JL PNR for that one flight. If so, try using the JL booking management web tool with that record locator. if that fails, try calling JL and giving them that record locator. You can also try using the Qantas or Finnair websites to manage the booking, using either the overall record locator (same as for JL) or the limited services one. You can also try calling AS back or using their website to allocate seats,using the AS record locator for the overall itinerary (which will be different from the JL one).