The iMac 27" Retina 5K Hard Drive Combo Cable (14/15) - Apple P/N 923-00092 cable is the SATA & power cable for the HD of the older model (late 2014 through Mid 2015 iMac 27"). The rub here is the length long enough to fit your system, that is if Apple altered the location of the headers on the logic board.
Don't forget you'll also need this special in-line thermal sensor: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later
But before you go down this path are you sure you want to do this??
Once you open the system Apple will wash its hands of any problems within the system (no longer covered under AppleCare). Given this is such a new system I would recommend you wait before you do this.
The other factor here is removing the display is a bear! All of the 'Thin Series' are. It's quite easy to damage the display and very expensive Oops! If you don't use the correct tools and follow the IFIXIT guide on how to open the system.
Here's the IFIXIT guide you'll need to follow: iMac Intel 27" Retina 5K Display Hard Drive Replacement.