The app is the easiest to just grab a checkin time. Just go immediately to Arrival Time and select a time. Scroll down further on the page and select a Arrival Time for your spouse also.
You can come back at any time -either via the app or web and complete the remainder of the checkin information.
You should be able to check in at 12:01 departure time of the port. So if Fort Lauderdale - Midnight Easter on 9PM Pacific