Never been able to do it. But you can checkin online.
I find CX to be an excellent airline, one of the best, but their website is lagging most other leading airlines in their booking and manage-my-booking functions. Although fsuch unctionalities are already below par for MPC, they are mediocre for non-MPC. They should realize that HK is an international hub and that many CX pax are not MPC. Such a functionality is not a "perk" for MPC, it only reflects the current below-par level of their website and IT.
I know that this comment will attract a flurry of angry responses from CX loyals, but I believe that I have good experience with CX & other airlines (I do my own travel management,mostly online).