Can I format my computer to Windows 10 without using the product key?

I have a Lenovo laptop that had Windows 8 preinstalled on it, and I upgraded it to Windows 10 while it was free for Windows 7/8 users. I would like to format the laptop to Windows 10 so I can sell it, but I can't find a way to reinstall Windows 10 without any sort of product key. Is it possible to format my laptop to Windows 10 without a product key?

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