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