In the rich text editor:
@tommy1
In a comment:
[tommy1](https://chowlings.com/user/tommy1)
For Names and Avatars:
In the rich text editor:
%tommy1
In a comment:
[![tommy1's Avatar](https://chowlings.com/images/avatars/default.jpg)](https://chowlings.com/user/tommy1) [tommy1](https://chowlings.com/user/tommy1)