Contrary to all logic, you have to start by visiting
https://contactless.tfl.gov.uk/ and signing up for an account if you don't already have one, then click on "Oyster Cards" in the "My Account" menu on the right-hand side of the page, then on "Service Delay Refunds", at which point it gives the option to select that you hold a paper ticket.
When entering the ticket number, include the 10 digit number with the issuing location details as well (for consistency with the previous paper forms). For example, ticket 12345 issued on machine 4746 at Window 1 at St Pancras might be: 12345 4746155501.