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