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