There are a few places you can find your booking reference.
In the confirmation email
You’ll find the booking reference in the subject of your confirmation email.
On the “Ticket details” screen
For all bookings, both etickets and paper tickets, you can find the booking reference at the bottom of the “Ticket details” screen.
For paper tickets
If your booking is for paper tickets, you can access your booking reference directly from the “My tickets” tab list of tickets and from towards the top of the “Ticket details” screen.