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