Hello.
I wan’t to buy a Win 8.1 for my PC and upgrade it to win 10 and my question is gonna Forza Horizon 3 work on upgraded version 8.1 for win 10 ? Or it required a default installed Win 10 on PC.
Thanks for Answer 
It doesn’t matter how you get windows, you just need to be on windows 10. So go ahead with whichever option you like.
Yes.
Depending on your Win8 and/or Win10 key (OEM, SB, etc.) …
a) you can install Win10 direct without Win8 first
or
b) you need to install Win8 first, to be able to upgrade to Win10
Anyway.
After that you end up with a standard version of Win10, no matter if ‘upgraded’ or ‘installed direct’.