In the rich text editor:
@sys632
In a comment:
[sys632](https://chowlings.com/user/sys632)
For Names and Avatars:
In the rich text editor:
%sys632
In a comment:
[![sys632's Avatar](https://chowlings.com/images/avatars/default.jpg)](https://chowlings.com/user/sys632) [sys632](https://chowlings.com/user/sys632)