You need to give them a signed message.

You can download this website and use it offline to sign the message: https://bitcoinmessage.tools/ (do not use it online). It is made by a Bitcointalk member. https://bitcointalk.org/index.php?topic=5440095.0

Next you need to locate the address you used to pay for copper membership. Do you remember this address? if you used a wallet application, you should be able to export the private key, and use the above website to create a signed message.