I understand that you would like to sign up for the iPhone Upgrade Program, but you are concerned because you do not yet have a carrier that you use. I'm happy to provide some help for this.
You can find out all about the iPhone Upgrade Program from the site below. I've highlighted the section that talks about your freedom to choose your own carrier:
Works with your carrier.
You’re buying directly from Apple. Which means no more financing your iPhone through your carrier or committing to a multiyear service contract. Your new iPhone will work with your carrier, so you won’t have to make any changes to your rate plan. And if you ever decide to switch carriers after you activate your iPhone with AT&T, Sprint, T-Mobile, or Verizon, you can easily do that, too.
Thanks for using the Apple Support Communities. Cheers!