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