Can you not do that?
You definitely can't click that handle and add follow them, I could put https://cooldude.example.com/@cooldude and you could click to their profile on their instance, but there would be no way to follow them directly from there on your mastodon.social (or whatever) account, you have to go to your instance and type/paste in their handle and then click follow.
Edit: There is a remote-follow workflow that involves clicking a link and entering your full handle to get redirected to a page where you can follow them. Maybe slightly better, but still not great.
I mean, I'm not saying that mastodon is super duper easy to use for everyone but in this particular example you seem to be going out of your way to make it more complicated than it is.
If the thing you found on hn was a link to their profile your flow would make sense, but if you have an id@domain the flow is really simple and basically the same as other similar things (ie. put it in your email client).
With email addresses, you have the mailto: link, which works pretty well so you don't have to copy paste. Maybe we need a follow: url scheme that browsers know what to do with.