This assumes you are eligible for hardware based entitlement to Win 10 PRO
Enter the default Windows 10 Pro product key:
VK7JG-NPHTM-C97JM-9MPGT-3V66T
Below message is shown

Click Start Upgrade, and after a few minutes, the system restarted. As soon as I connected to the Internet, the Windows activation process checked my hardware against Microsoft’s servers. Because this hardware was already recorded with a digital entitlement to Windows 10 Pro, I was activated immediately, without the need to enter a product key.