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