It can be done, IF the PCL rep is trained!
I had such a hard time trying to do if for my last cruise. 3 agents, all tried, the last one did it, but had to first correct the mess of the 2nd agent, who doubled the GC and then it all disappeared completely and this agent had to find it and reenter it. What a mess, but, finally it was in.
They have to do it in another screen and they have to know the code and all the entries. It is not an easy transaction for them. Some refuse to even try. So, if you want to do it pre cruise, I’d suggest contacting your TA/CVP to do it for you.
Make sure you check your travel summary and see it! Print it out! Hold onto the GC info until your cruise is complete.
If not, just take a printed copy of the certificate to GS. It took them about 5 mins tops! Never again will I waste my time pre cruise. Lesson learned.