Activating Windows Server confirms your copy is genuine and unlocks ongoing updates and support. Here's how to do it after receiving your product key.
Activate from Settings
Open Settings → System → Activation (or Server Manager on older builds).
Select Change product key and enter your 25-character key.
Windows validates the key with Microsoft's servers and activates within a few seconds, provided the server has internet access.
Activate from the command line
On a server with no GUI, run slmgr /ipk <your-key> to install the key, then slmgr /ato to activate. Use slmgr /dli at any point to check the current license status.
Confirm activation
Go back to Settings → System → Activation — it should show "Windows is activated."
