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