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