Unfortunately this feature is only available on the app at the moment. If you would like to repeat a booking, just go into 'My Bookings' and open past booking> scroll down and click on 'Repeat Booking'. Then select date and time.
If the selected trainer is available at the chosen time, they will confirm the session. If they are not available, they will either respond with an alternative time or decline the session. If they cannot do it, the booking will be send out to other trainers in case you still would like to have a session and would not mind training with someone else.