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