You can’t, but there’s no need. You can read posts to communities on other Lemmy servers from your account on lemmy.ml. You can subscribe to those communities and comment on posts there as well. There’s no need to have an account on each server. It’s like email. You don’t need an email address on each server to send/receive email. There may be a reason to have another account - maybe you get really involved in a community on that server and that server feels more like home. But, no, there is no protocol for cross-authentication.

Searching for the URL allowed me to find it and then subscribe from my instance. Thanks for the assist!

Seems like a flaw that I couldn’t search for photography@lemmy.ml, though.