Hello! I bought a flight itinerary from SIN to EWR via HND and LAX, with the LAX-EWR segment operated by AS. On the JL website, I was able to select seats on the SIN-HND-LAX segments as usual, but it seems difficult to input the confirmation code via the AS website, resulting in errors. My intention was to select seats and purchase some in-flight services on the AS flight, so it is disheatening if I cannot be able to do these online.

Is there some method I can get the AS booking modified and use the online services, or am I only able to call the JL number for something like seat selection at least? I read about that there is a hidden confirmation code from the operating airline - does AS have this for JL-ticketed itinerary?