As of 2025, for Windows Server 2008 R2 Standard. Running it on a network connected to the internet is a significant security risk.
If you installed without a key or used a trial key: product key windows server 2008 r2 standard
If you buy Windows Server 2022 Standard, your license includes to prior versions, including 2008 R2. You would use the Windows Server 2008 R2 Standard product key provided by Microsoft for downgrade scenarios (available via Volume Licensing). As of 2025, for Windows Server 2008 R2 Standard
Last Updated: 2025 – Information is provided for educational and legitimate licensing purposes. Unauthorized use of product keys is illegal. You would use the Windows Server 2008 R2
Microsoft does not offer a "lost key" retrieval service for end users. If you have no key and no activated server, you must purchase a new license (though buying 2008 R2 new is no longer possible; you would need to buy a newer version with downgrade rights).