If you have the private key of your wallet then you can use that private key to sign a message. Maybe you have seen the PGP keys out here in this forum before, those are encrypted messages which can be viewed with your public key but can only be unlocked or in this case signed from your private key. And as we know private keys are what truly makes you the owner of the Bitcoin address.
https://bitcoinmagazine.com/articles/bitcoin-address-sign-1399914228/
Try this article to find out more about how this signing addresses works and how to perform them.