If you are referring to the Windows 10 product key, Microsoft's digital license called digital entitlement doesn't require a product key. Windows is activated when you connect you computer to the Internet. If you go to Control Panel / System and Security / System you will see you Product ID and an option to change your Product Key, but I see not reason to do that.
product keys are stored in the BIOS. There are tools to extract the key, as the key is the Product ID shown.
I extract the key and write it on the inside cover of the case just in case I need it in the future.
Thank you for your message.
Windows 8/10 has digital license & the OS will be activated automatically when connected to the internet. There is no separate product key card shipped with the computer.
The activation key is injected to the motherboard, you will not be prompted for an activation key when you setup your Dell computer shipped with Windows 8/10.
If there is a hard drive replacement or OS reinstall done with a Dell image, you will not require the activation key as it will be taken from the motherboard.
Only when the motherboard is replaced the product key changes. If the board is replaced by Dell there will be a new product key embedded on the replacement board.