In the rich text editor:
@Haruki
In a comment:
[Haruki](https://chowlings.com/user/Haruki)
For Names and Avatars:
In the rich text editor:
%Haruki
In a comment:
[![Haruki's Avatar](https://chowlings.com/images/avatars/default.jpg)](https://chowlings.com/user/Haruki) [Haruki](https://chowlings.com/user/Haruki)