I don't think you can. But you should be able to start the service, wait until you get the phone and then promptly cancel it. It'll make the phone "due" and that should yield a phone without contract in the end right?
It shouldn't be "due" if the unit is paid off for.
But it appears at the very least, you need to be able to buy atleast 1-month of service to get a S6. Which sucks because thats a $50 service I don't need and I'm already paying $950 in the end after tax. So its $1000 for whats listed as $860 on the site.