Alternatively, you should be able to use a good 'ole Windows 7 Pro OE [or] Windows 7 Enterprise key to activate.
It's mostly unknown to consumers, but a Windows 7 key can still be used to fully activate a legitimate digital license of W10, vis-a-vis. The only caveat is: You may just have ISO/Mount [or] Extract down and install your build rev (must be same or newer), in order to do it. Alas, it saves you a $99 upgrade fee, and for those of you whom hate the constant patch-Tuesdays for Windows 10 and would be overcome with glee if there was a "No-ring" option for update frequency, the aforementioned info may be of use.