In the rich text editor:
@5u2k
In a comment:
[5u2k](https://chowlings.com/user/5u2k)
For Names and Avatars:
In the rich text editor:
%5u2k
In a comment:
[![5u2k's Avatar](https://chowlings.com/images/avatars/default.jpg)](https://chowlings.com/user/5u2k) [5u2k](https://chowlings.com/user/5u2k)