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