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