Once your bookings has been confirmed you'll receive a confirmation email with all your trip details and booking number.
Remember to check your spam and junk folders if you can't find it.
You can also check this information loging in to Manage My Booking, there you will have the option to request the confirmation email again.
Note: This information applies for bookings with 10-12 digits. If your booking has a different format, please refer to your confirmation email, where you will find the way to contact your hotel provider.