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