Should you require to extract the product key from pre-installed MS Windows 10

There is no need to know the product key, since the machine has a digital entitlement. 

 If you need to reinstall MS Windows due to a reason out of many, you may think that you require the product key of the preinstalled with Windows 10. Really you don't require the product key. what happened is the hardware (your PC) will get a digital entitlement, where a unique signature of the computer will be stored on Microsoft Activation Servers.

Anytime you need to reinstall Windows 10 on that machine, just proceed to reinstall Windows 10. It will automatically reactivate. 

If you are prompted to enter a product key, click 'I don't have a key' and 'Do this later'

If you want to really find the product key:

1. Open the command line as the administrator (Run As Administrator)
2. Type the command:
• wmic path SoftwareLicensingService get OA3xOriginalProductKey
  
3. Hit enter, the product key will be revealed
4. Copy the product key to a text file.