Within the envelope there will be a sticker that contains your product key. In most cases it has a silver scratch pad on top of part of the key to prevent unauthorized use of the key. Scratch that carefully to reveal your key.

More information can be found at:
https://support.microsoft.com/en-us/help/10749/windows-product-key.

Also see our article:
Where is my product key for Windows 10 OEM versions.