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