I've been through this a few times on my own, some really vague high level support chats (one of which said my hardware didn't support it yet?), and a call with a helpful senior support guy at apple (though he was a cpu guy for some reason and not really up to date on recent bootcamp specific changes).
Bootcamp has an acclimatization screen, continuing on from that I can select a Windows ISO image and slide my desired partition size, then hit install and the process goes on to download support drivers and then my laptop reboots. That's it. While not listed as a machine that bypasses having a USB flash drive on (https://support.apple.com/en-us/HT201468), I have no familiar "select task" screen or any prompts about external media. The bootcamp installer happily downloads support software, then I reboot into a blue screen http://imgur.com/a/c2dFY. Once back in macOS (10.12.1, Bootcamp Assistant Version 6.1.0 (6050.20.1)) the bootcamp partition is empty.
I'm assuming that for whatever reason the contents of the ISO aren't being copied onto the internal partition, hence file not found and install hangs immediately.
For kicks I tried using a random file renamed as an ISO and I get the following: "Boot Camp only supports 64-bit Windows 10 or later installation on this platform. Please use an ISO file for 64-bit Windows 10 or later installation." So it is recognizing my image as valid.
Trying to do things solely from my machine I assume I'd need either:
1) To make my own bootable Win10 USB Flash drive. All the instructions I find say to use the bootcamp assistant, are for creating bootable OS X / macOS installers (createinstallmedia), or use windows tools to create the drive. I'm assuming a simple FAT32 partition with GUID table won't work?
2) To find out how the 2015 mac products work and mimic that once the partition is created, install process fails, and I'm back in macOS.