Windows 10 on USB or HDD

Hi,

I would like to install Windows 10 on an External HDD or a USB Drive (128 GB) and run it via Bootcamp, i have no past experience with bootcamp as i am new to Mac, however i would like to run a few of my favorite games which are not supported on Mac.

My question is: am i capable of installing Windows 10 on my USB Drive or External HDD and boot from it?

I have a: MacBook Air (13-inch, Early 2015) running OS X El Capitan