warranty without a receipt?

Hi

The One-Year Limited Warranty is provided automatically, with no need to register for it. Apple can check the coverage period from your watch serial number (on the back of the watch).

To obtain service under warranty, you may be required to provide proof of purchase, normally via the original receipt or a gift receipt - so try asking the competition / prize draw organiser to provide this to you. This is a user-based technical support community, so no-one here can tell you whether anything else may be acceptable. You can ask Apple either by visiting an Apple Retail Store or contacting Apple Support:

https://www.apple.com/support/watch/contact/

If you wish to buy AppleCare+ (which provides additional hardware coverage and cover for accidental damage), this can be done within 60 days of the watch's original purchase date. Proof of purchase is normally required, so Apple Support can again help you further.