invoice without an invoice?

Hello, I want to apply for https://www.jugendherberge.de/en/membership/eurail/ German Youth Hostel Membership - but there is a “Please submit a copy of your Eurail invoice as a “PDF file”.” And I did not pay for my pass - it was granted to me as a part of a programme. How do I get an invoice or some other document that they will maybe accept?