In the rich text editor:
@Yuusuke
In a comment:
[Yuusuke](https://chowlings.com/user/Yuusuke)
For Names and Avatars:
In the rich text editor:
%Yuusuke
In a comment:
[](https://chowlings.com/user/Yuusuke) [Yuusuke](https://chowlings.com/user/Yuusuke)