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