Does the current Windows 10 Pro show it is activated with a digital license or digital entitlement on that computer (it does not have to be linked to a Microsoft account - just a digital license or entitlement).
If it does say digital entitlement or digital license, and you want to clean install all you have to do is:
1. Boot from Windows 10 installation USB flash drive or DVD.
2. Skip entering product key if it asks for one.
3. Select the Custom Install option.
4. You will be presented with a list of partitions on the hard drive/SSD. Select each partition on the hard drive/SSD you want to install Windows 10 to. This will erase everything on the hard drive/SSD.
5. Select the resulting unallocated space and click next to let Windows set up the partitions and install.
It is recommended that you have only the hard drive/SSD connected that you want to install to. Disconnect all other hard drives/SSDs (except for the Windows 10 installation USB flash drive or DVD of course).
Windows 10 will activate itself by retrieving the digital entitlement or license from Microsoft activation servers after it is installed and connected to the internet.