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