If you are on subscription you can log in the Subscription portal and find your serial numbers in the coverage report or your contract administrator will have it on record. Otherwise your reseller should have it on record for you. It is also attached to your media; yellow portion of the white sticker on the back of the media case. There are many other places but these should lead you and most customers who might read this thread to it or someone who has it.
Douglas Lauritsen, Autodesk Product Support
Douglas Lauritsen
Support Specialist
Product Support
Autodesk, Inc.