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