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