You will find hundreds of YouTube videos telling you to open Command Prompt and paste a long script or download an "Auto KMS" tool.
Note: This will only work if your PC has a digital license already linked to its hardware. If it is a brand new PC, this method will fail.
If you use Windows every day for work or gaming, buy a legitimate OEM key (they are often $15-$30 from reputable third-party resellers, not Microsoft's $139 price tag). It removes the watermark and ensures you get all security updates and personalization features.
The short answer is . You can install and use Windows 10 or 11 without ever entering a product key. However, "activating" it without a key is a different story.