It’s easy enough that you can do it inadvertantly. It’s the field at the bottom of the reply screen that says “tagline.” Just click inside the field and type whatever you want there, within the limitation of the length of the field.
I must’ve hit the tab key by accident or something, because I popped up with a plus sign in mine, just noticed it this morning. Sort of like it, so I think I’ll leave it for a little while at least.
I like your (+) too!
It’s simple and will likely make people wonder what it means ; )