I have a Canadian Amex Plat and would like to use the $200 travel credit.

The problem is, when I book online through travel.americanexpress.ca (CX YYZ-HKG-NRT), it only offered me only one fare class (CX fare class S), which is too restrictive for me.

Is it possible for me to call in and ask the agent to book me on a different fare class and use the $200 credit over the phone?

I checked on the CX website and I can confirm they still have the other fare classes available.