Depends how much you can afford to lose. The fact is that many of the keys on Kinguin are stolen or sold in violation of the governing license agreements. Often you will get a "working" key, but when it comes time for its 6 month check in with the Volume licensing server of the company it was stolen/illegally resold from, it will fail, since you are not part of that company.
Kinguin's keys generally last just about long enough that they have no liability - one of the reasons they're notoriously cheap. You might get a working key, but I wouldn't bet on it, and so if you can afford to gamble be my guest, but otherwise you should use a legal key.
It's worth noting that you can use windows 10 legally without a license key, but it does...