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