1. Visit https://al-support.apple.com/#/kbase
2. Scroll down and choose 'For a device owned by a business, school, or institution' under Need additional support.
3. Agree to the t&cs and click 'Continue'.
4. Enter your email address and the serial number of the locked device. Then click continue.
5. It should then send you a link to sign into your apple account and upload proof or purchase.