http://lxqyom6a3p6fdgsy6mpjyhjqjdpzleseck2jojwwzcilmsvduaeo7zyd.onion/2020/06/11/how-to-prove-control-of-a-bitcoin-segwit-address/index.html
There are a number of online services that offer signing services, but all these come with the intrinsic problem that during the process you must disclose your private key, which is something you never want to do online (why we will not publish links to any such webpage). A good rule-of-thumb is that your private keys should never be transferred over any network. Private keys should be used for offline calculations only.