In the rich text editor:
@Shadow69
In a comment:
[Shadow69](https://chowlings.com/user/Shadow69)
For Names and Avatars:
In the rich text editor:
%Shadow69
In a comment:
[![Shadow69's Avatar](https://chowlings.com/images/avatars/default.jpg)](https://chowlings.com/user/Shadow69) [Shadow69](https://chowlings.com/user/Shadow69)