I tried this and it did not work for me. I booked main cabin on American and we have a code share flight on Iberia. I'm traveling with my 12-year-old and really want us to sit together, but I already paid extra for main cabin on American so I don't want to turn around and pay Iberia again to select seats when I've already paid.

When you went to Malaysian Airlines website, did you put in the American Airlines reference number or the Iberia reference number? I tried both and and neither one worked for me. I can't believe that I can book a fare that is supposed to include seat selection but still can't be guaranteed to sit with my 12-year-old on a transatlantic flight without paying even more.