Far as I understand, there isn't a way to do that. Federated usernames work essentially like an email address because of the decentralized nature of the service. If I type @sparky@lemmy.federate.cc it won't send you a notification, but it will link your profile.
At least I think that's how it works. Who wants to jump in and point out all the ways I'm wrong?
Not saying you are wrong, since it is currently imposible to do here, but I don't think the federated nature of usernames has anything yo do with if they receive a notification or bot since over at mastodon you can tag people properly and they are notified.
tl;dr answer: There's no tagging function in place unless you manually tag them using markdown, and that still wouldn't notify them. That would be a bandaid solution at best.
Which is strange because there seems to be support for notifications of a mention? Jerboa even has a section for it, but no one has mentioned me yet so I don't know if it works.
But my personal “Mentions” inbox just shows nothing (I have been mentioned before by other users but only noticed it while browsing this instance and not via inbox).
You can tag people using @user@instance.tld, but only when using the official web UI . Third party apps I tried doesn't seem to support it. For example, try tagging @remindme@mstdn.social to remind you in 2 hours.