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