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