Did you attach your rapid rewards number to your booking? If so if you log in to your account it should show up under the my trips section.
Alternatively you can call reservations at 800-435-9792 and they can help you look it up usually with the Credit card number you used for booking.
Also when you booked it should have sent you an email confirmation so I would double check your email for that (including the spam folder) in case it got misfiltered.
Hope this helps
Blake
