In the rich text editor:
@Ag_17
In a comment:
[Ag_17](https://chowlings.com/user/Ag_17)
For Names and Avatars:
In the rich text editor:
%Ag_17
In a comment:
[![Ag_17's Avatar](https://chowlings.com/images/avatars/default.jpg)](https://chowlings.com/user/Ag_17) [Ag_17](https://chowlings.com/user/Ag_17)