If you have an account in a Nextcloud instance and has added the application for its support (and I think that must also add a configuration but I didn’t check yet), you can use it there.

Mailcow also supports it, but I think only as 2FA and the other service I know is nonfree, so I don’t want to mention it.

Support is still the process but I think that some or most mainstream websites support it at some point.