Add private message link to comment

What would be the best way to add a "Private message" link such as the one available for nodes to a users comment?
I'm assuming a custom field would be the best way to do it. That way it's just adding the field to a structure type but I'm not sure how to link it to the users message link.

I've tried adding a message link to the comments(or anything else that uses fields) by
Going to /admin/structure/comment/manage/
Manage Fields
Add new Field > Reference > Other
Type of item to reference > Private Message Thread
But none of the references are for a link , they all display a form for username input(who you want to send a message to)

Thanks for any help :)