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