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