I can't boot Windows 7 USB from macOS Sierra using option key I want to install Windows 7 on my Mac, which is running macOS Sierra.

I can't do this with Boot Camp, since Boot Camp only supports Windows 8 and later.

I need to boot from the USB, which I created, but I can't do it using the usual method of holding down option key while booting the Mac.

I also tried holding down shift key, nothing works, it just starts as usual.

How can I actually boot from USB and are there different keys to press in order to boot from USB or boot in safe mode in macOS Sierra?
Mac computers using macOS Sierra 10.12 support new installations of
  Windows 10 only. macOS Sierra does not support new installations of
  earlier versions of Windows.


See https://support.apple.com/en-us/HT205016 for more information.