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