Im not sure you understand the problem. How does the server with the user you want to follow know what server to talk to if all you've done is click "follow"?
In that case, you can do it by selecting your home server. That's a search drop-down. Something like mastodon.social is there by default, and for historians.social, you might need to type in 'his' <click>.
I don't feel too bad about that either. It's like oauth, only instead of just having two options (Google and Facebook, typically), you have an unlimited options if you're willing to type a little bit more.
Or like SSO on many Microsoft web sites, where I type in my work domain or email, and it redirects through my work.
