If you have lost the product key for your Windows 11 Pro installation, you may need to contact Microsoft Support for assistance. They can help you verify your purchase and potentially provide a replacement key. Additionally, if you have a digital license linked to your Microsoft account, you may be able to install Windows 11 Pro on your new computer without needing the product key, as it can automatically activate using your Microsoft account.
Make sure to have any proof of purchase or other relevant information ready when you contact support to facilitate the process.