Actually someone pointed out you don't even need to do that much

There's a big (and only) search bar at the top of the homepage. You can simply search the username and they'll pop up across instances. There's a little follow button next to their name you can click and you're done

Exactly like Venmo or Snapchat or Instagram. It's already in the cultural norms to ask "what's your insta" or "what's your venmo".

What's your mastodon?