http://lxqyom6a3p6fdgsy6mpjyhjqjdpzleseck2jojwwzcilmsvduaeo7zyd.onion/2020/06/11/how-to-prove-control-of-a-bitcoin-segwit-address/index.html
To prove ownership over an address, sign a short message with it. It must be a message that you are willing to share. Simple enter the message in the Message box, the public address in the Address box and click “Sign”. It will populate the Signature box with the signed message, which is the string you will use together with the public address and message to prove control.