In the rich text editor:
@Pika
In a comment:
[Pika](https://chowlings.com/user/Pika)
For Names and Avatars:
In the rich text editor:
%Pika
In a comment:
[![Pika's Avatar](https://chowlings.com/images/avatars/default.jpg)](https://chowlings.com/user/Pika) [Pika](https://chowlings.com/user/Pika)