When installing a fresh Windows 8 Developer Preview, you do not need to enter any product key during installation. The product key is already included in the setup.
When product key can be asked
- If for some reason, windows 8 installation was interrupted and you have to re-start installation process, you may be asked to enter the product key.
- If you want to reset your windows 8 using Refresh or Reset functionality, you may be asked to enter your product key.
Product key of Windows 8 Developers Preview
Product key of Windows 8 Developers Preview – Server Edition