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#21
If you remember your Windows version (Home or Pro), you can use latest ISO to start the installation. At the key prompt screen click the I do not have a key option. This will allow you to proceed without the key and install Windows 10. After installation is complete you can go to activate the license and will prompt you a key. It might also activate automatically once online. You may even never be asked for a key if this is stored in the firmware (BIOS).
PS: If you need to see the key before installing Windows 10, you can either boot with a WinPE CD or USB, or connect your hard disk to another computer. Then you run the ShowKey Plus utility to see your Windows 10 key.