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