I used the following procedure to extract the Windows 8 key from a Lenovo desktop.

Download RW from: http://rweverything.com/. I used the 64-bit portable version.

Copy the Win64 directory from the zip to your local drive.

Execute the RW.EXE program in the Win64\Portable directory.

Once open, click the ACPI icon or select ACPI Tables from the menu.

Then, click on the MSDM tab in the ACPI Table window.

You will find the key in the bottom of the ACPI Table window.