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