If the key is genuine and not used on another PC (or the old PC is dead), it will usually work. Microsoft calls this a "free upgrade" loophole for accessibility.

If you search "how to activate Windows 10 Pro without product key," you will find YouTube videos showing how to use , KMSPico , or CMD commands to bypass activation.

If you’ve just installed Windows 10 Pro and are staring at the "Activate Windows" watermark in the bottom-right corner, you might be searching for a way to get rid of it without immediately entering a 25-character product key.

Windows will automatically contact Microsoft servers, recognize your motherboard's hardware ID, and activate itself.

If you bought a retail (not OEM) copy of Windows 10 Pro, you are legally allowed to move it to a new PC. You don't need a new key; you just need to transfer the license.