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If you own Minecraft and are using Windows 10, Mojang laid out some instructions for you to follow on how to obtain your key for your "free" copy. If you run into an error, don't panic. I ran into an error after I tried to obtain my key. Simply go back to your Mojang account page and it may list your key there. Take that key and redeem it through the Windows Store app in Windows 10. I wouldn't suggest trying to redeem it on Windows 8.x, just in case.
Also, for those that are wondering: Yes, this version of Minecraft does indeed include achievements. I believe they are the same ones as the Xbox versions of the game.