How to add a comment with a referral to the user whose username has a space inside the string? (@) Lets say an user "John Doe" commented under some answer post. How to write a comment that will contain a referral to John Doe's username? (referral by using the @ symbol). I've tried: @(John Doe) @John-Doe @JohnDoe Unfortunately none of them worked for some reason.

@autocompleting was implemented some time ago for comments. You should be able to type @ followed by the first character of the username and you'll be presented with a list of suitable usernames to tab through.

!@autocomplete in action

In the example above BillWeiss real username is Bill Weiss so just removing the space(s) should also work.