How do I get future cruise credit if I have a comped cruise?

I have a fully comped cruise, how am I getting the FCC (future cruise credit)? In the case of a fully comped cruise, you would be refunded your tax and port fees. If you didn't pay any money in the cruise fare, then there wouldn't be any future cruise credit to receive. If you paid for a third or fourth guest or paid the upgrade your cabin,  those costs would be cruise fare, and that would be an option for you to take for rebooking.