To send a mention use type @user@instance and select from the dropdown box, alternatively type the full link manually [@user@instance](https://instance/u/user)

You can generate an instance agnostic link with /u/user@instance automatically, which will allow other users to see their profile from within their own instance, however this won't give a mention notification to the user.

I think something like [email protected] would be more the ticket, but I know that Memmy doesn’t have an option in the inbox for either mentions or messages, just replies, so you might not see it depending on what you use to access Lemmy.