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