I have tried to find another thread on this topic but couldn't find one. I have an upcoming trip going from YYZ-YVR-HKG-CGK booked through Air Canada. The last leg is booked on a code share with Cathay Pacific. During the booking process, I was asked to select a seat on the Cathay flight but none of the seat selection shows on the final itinerary. Instead, it states that I must make my seat selection on Cathay's website. However, for the life of me, I cannot locate Cathay's booking reference on the itinerary sent to me by AC. I tried using both the website and the app but still cannot find it. Has anyone else run into a similar issue? I would call AC reservations, but with all the disruptions, I would prefer to DIY this if I can.

Cathay uses Amadeus, so it's the same reference.

Even on airlines that don't use Amadeus, the Amadeus reference will often work.

You could try calling Cathay. In general, even if you don't have their native reference, the eTicket number will work on the phone (and often on the website).

I haven't flown CX on a 014 ticket since the Amadeus switchover, but just using the ticket number to pull up the booking worked before.

Sorry to dig this up but similar issue here. HKG ICN YYZ, HKG ICN is a codeshare with CX (more specifically it's actually an interline flight with CX).

When I added the Amaedus PNR to my Cathay one world account, it found my booking. However, I wasn't actually able to select seats since apparently my ticket is as "Economy Light" (which according to CX's site, isn't entitled to advance seat selection).

Selecting seats through AC's site (through "Change Seat" under "My Bookings") results in "we are unable to select your seat for this flight".

Does this mean I'm screwed (I booked the ticket as PY lowest ticket, but I presume seat selection/etc. terms are set by the operating carrier?) And for future reference, am I supposed to be selecting seats through CX or AC's site?

I plan to purchase Business class international ticket through Air Canada website, the flight is operated by Cathay Pacific or Eva Air.
Can I choose the seat at the time when I buy the ticket?
Thanks!

With Eva yes.

For Cathay you have to go to their site and enter your booking code to choose seat.