How do I get my Windows 8.1 Product Key?

Option 1: Windows 8.1 came preinstalled on your computer:

With Windows 8, Microsoft has changed from stickers that have the product key that the user has to type in when installing the operating system to new BIOS embedded product keys. The idea is that by eliminating the sticker, you eliminate one of the easier ways for nefarious users to get a legitimate product key. Eliminating the product key sticker also removes any worry that the sticker might get damaged while at the same time eliminating the long and irritating process of typing in various letters and numbers when installing the operating system.

If the user has to reinstall the operating system on a machine that came with Windows 8, the installation process automatically grabs the software product key from the motherboard BIOS with no input from the user. This means that those familiar Windows product key stickers will no longer appear on the Windows 8 computers.

Microsoft has noted that persons who upgrade Windows 8.1 installations to Windows 10 Previews, this will make their recovery partitions inoperable, requiring that you have recovery media to do the actual reinstall. If not, you have to utilize the procedures in the following article:

Option 2: If Windows 8.1 was bought online:

Go to the order summary page, there you will find your product key:

https://www.mswos.com/

Go to this website and click on "Account/ Download + product keys " (near the page bottom), the information there should help you obtain your product key and order information.
http://www.microsoftstore.com/store/msus/en_US/home