If you purchased Media Center, you should be able to find your product key in your email inbox.
I don't think anyone here can give you a key as it is purchasable software I believe.
If you are running Windows 10 Home/Windows 10 Pro, you won't get Media Center as Microsoft dropped support.
Bear in mind, if you are running Windows 8.1 Pro with WMC or looking to add WMC to 8.1, if you upgrade to Windows 10 on or after July 29, you will lose Media Center.