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