If you have an old computer lying around with a valid Windows 7 or Windows 8 license sticker, you can legally use that key to activate Windows 10 or 11. Microsoft technically ended this free upgrade offer years ago, but the activation servers still accept these keys in most regions.
If you are a student or work for a large organization, check with your IT department or school. Many universities partner with Microsoft to provide genuine keys to students for free or at a massively reduced cost through the Microsoft Azure Dev Tools for Teaching program. how to activate windows without a product key
But you can Windows without activation forever. If the watermark doesn't bother you, save your money. If it drives you crazy, buy a legitimate key. If you have an old computer lying around