Windows 10 on Mac book Air

[pinchez]

I have a MBA that I've just sold with a Windows 10 partition, if I wipe the SSD and just put OSX back on will the new user be able to install windows 10 without a key later?

I originally installed Windows 10 pro with a genuine key which I can't find

 

[Tarek]

I am not 100% sure, but logically speaking, the new user will need a product key to install Windows 10.