When making your priority 1 booking, only fill up the details of the one-way transport service.
Tick the box beside the service that you wish to book onto. Click "confirm" to confirm your one-way transport service. You will be asked to confirm your decision to book a one-way transport service, click "Book one way trip" to confirm.
Note that in Priority 2 bookings, you will not be able to add the other direction once you have confirmed.