A horn button typically provides a ground for the horn. On a two terminal button, the ground is the second terminal. On a one terminal button, the button case itself is the second terminal since it is intended to be installed on a grounded metal horn ring or steering wheel.
You can fiddle around with the hookup with no danger. If your horn is properly wired, when you hook up the button wrong, the horn blows – no biggie.
Hope this helps.