In the rich text editor:
@user1234567
In a comment:
[user1234567](https://chowlings.com/user/user1234567)
For Names and Avatars:
In the rich text editor:
%user1234567
In a comment:
[![user1234567's Avatar](https://chowlings.com/images/avatars/1996.png?v=1676734220)](https://chowlings.com/user/user1234567) [user1234567](https://chowlings.com/user/user1234567)