Can I bootcamp my mac with only a product key from windows8.1?
You can download the mediacreationtool.exe which will allow you to download the .iso or create the USB flash drive.

I believe you want an iso. If you use the mediacreationtool to create a USB flash drive, it will not work on a Mac. You need to burn the iso to a DVD or use Boot Camp Assistant to create the USB.

Of course you will need to execute the mediacreationtool from a machine booted to Windows. If you do not have such a machine, you can install a temporary evaluation copy of Windows Enterprise 8.1. The iso can be downloaded from here.

To be clear. You download the 64 bit Windows Enterprise 8.1 as an iso. You use the iso to install Windows to your Mac. Boot to Windows and download the Media Creation Tool. Run this tool to download a iso of the Home or Pro version of Windows 8.1. This iso should work with your product key. You then erase the installed Enterprise Windows and install the Home or Pro Windows using the Product Key.


  Note: For almost all Macs, you want the 64 bit version of windows.