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