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