The return time slot cannot be specified. If you wish to book a specific time slot, please purchase at the following links:
English booking page: https://sg.trip.com/travel-guide/attraction/singapore/tanah-merah-ferry-terminal-13415873?poiType=3&locale=en-SG
Chinese booking page: https://sg.trip.com/travel-guide/attraction/singapore/tanah-merah-ferry-terminal-13415873?poiType=3&locale=zh-sg