Where do I find more Window 8.1 Product key?

Hi, I have brand new Dell (less than a week old), preinstalled with Windows 8.1 and what to ensure I have a USB installer ready to rebuild the system if required (I've had a lot of issues). Where do I find the Windows product key? 

This sort of thing used to be on a nice sticker on the machine, but due to 'progress' I'm having to ask this online. Help!

Thanks