Unknown. Apple has not made any info public regarding the requirements to choose a carrier for the Apple Card Monthly Installments
With that said, you can select a carrier as requested and once you have the iPhone in hand, bypass the activation with the chose carrier. T-Mobile appears to be the easiest one to do this with, at least in the past it has been.
If you can borrow a T-Mobile phone line from a friend or relative you can select T-Mobile and then activate the iPhone with your own carrier. This will not affect the phone line or device you borrow it from for the purchase process.
click here ➜ Unlocked iPhone Apple Card Financing - Apple Community