In the rich text editor:
@OLD_USER_001
In a comment:
[OLD_USER_001](https://chowlings.com/user/OLD_USER_001)
For Names and Avatars:
In the rich text editor:
%OLD_USER_001
In a comment:
[![OLD_USER_001's Avatar](https://chowlings.com/images/avatars/default.jpg)](https://chowlings.com/user/OLD_USER_001) [OLD_USER_001](https://chowlings.com/user/OLD_USER_001)