You don't actually attach a button to the headstock (although I suppose it's possible). You tie the end of the guitar strap to the headstock with a something similar to a shoelace.
I have a guitar strap on there now that ties to the headstock, and has a quick release buckle so I can take the strap off for storage.
I'm thinking of adding a button to the heel of the neck, though, because I think it would sit better. I don't like the way the strap sits right now.
John