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:
[](https://chowlings.com/user/user1234567) [user1234567](https://chowlings.com/user/user1234567)