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