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