XF 1.4 Add to user message

There is no "Tag" option.

It's not clear what you're asking.
I will try to clear it up.

I would like to add a new option to messages, I would like to make a "tag" option. This will be next to the existing ones Like, +Quote and Reply. I want to know what kind of code I would need to add the word "tag" and when you click it in the reply box it puts "@Username" The username is the username of the person's post which you pressed tag on.