You just book a regular reservation . The only difference is the SSR, OSI code . The airline has to code the reservation with the DEPU. It’s for example if someone requires wheelchair assistance. It’s WCHR, or immobile, WCHC. But DEPU wouldn’t be available for a passenger to do online .You’d make the reservation, then call the airline directly. In the flight paperwork, it would indicate the passenger is getting deported.