I should clarify:
During Windows installation, when it asks you for a product key, skip that step. Choose 'Skip', 'Not now', 'Later', whatever it offers (just don't cancel). At that point, the only thing Windows will take is an actual Windows 10 product key. It's not until Windows is fully installed and connected to the Internet will a Windows 7/8/8.1 key work because only then will it have connectivity to the activation servers. Windows 10's ability to take older keys is for a limited time only, I think until June or July of this year, so it's not baked into the original ISO install image.
That said, you still need to know ahead of time which ISO image you need. IIRC, I believe there are two but I'm not sure how it splits them. There is either one with 32/64-bit Win10 Home and one with 32/64-bit Win10 Pro, or one with 32-bit Home and Pro and the other 64-bit Home and Pro. It's been a few months. Sorry.