Macbook 12 " and Windows 8.1
I had the same problem. I was installing Win 8.1 by bootcamp on a MacBook 12" early-2016. The internal keyboard and trackpad did not work.

I was using the USB-C to 3 ports adater with only one standard USB. So I tried to unplug the installation flash drive and plug a usb mouse. It worked, though obviously I cannot continue to install without the flash drive. So option 1 is use a USB hub and a external mouse.

Another option is to install Win 10. The internal trackpad works in the installation screen.