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