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You cannot book cars directly into to that cars calendar. You must invite the car into an appointment you make in your calendar.

1) Create your appointment

2) Go to 'Scheduling Assistant' up the top

3) Click on 'Add Rooms' down the bottom

4) Find and add the room/car

5) Save and send your appointment

The room/car will respond within a minute or two to advise that the booking has been successful (or clashes with another appointment).

This is to ensure that there are no 'ghost' appointments which aren't attached to anyone, and so cannot be rescheduled.