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