In the rich text editor:
@yugioh
In a comment:
[yugioh](https://cometcrashers.org/user/yugioh)
For Names and Avatars:
In the rich text editor:
%yugioh
In a comment:
[](https://cometcrashers.org/user/yugioh) [yugioh](https://cometcrashers.org/user/yugioh)