You can find it in your payment confirmation e-mail. It is a combination of 6 characters (letters and numbers - eg; AB12C3).
This starts with R followed by 10 or 11 numbers. ie R2020123456. You can find it in the confirmation email you received after booking. If you booked with another company, please contact them directly as we will not be able to help you.
A member of our support staff will respond as soon as possible.