A Windows product key is a 25-character code that activates and licenses Windows. You can find your product key in the confirmation email you received after purchasing Windows, on the packaging, or on your device. You can also use PowerShell to find your product key. 
Where to find your product key
Confirmation email: The email you received after purchasing Windows 
Packaging: The box that Windows came in 
Device: A sticker on your device if Windows came preinstalled 
Email inbox: If you purchased Windows online, the product key might be in your email inbox 
Retailer's website: If you purchased Windows online, the product key might be on the retailer's website 
Finding your product key using PowerShell 
Right-click on the Start Menu
Select Windows PowerShell (Admin)
Type or copy and paste the following code into PowerShell: powershell "(Get-WmiObject -query 'select * from SoftwareLicensingService').OA3xOriginalProductKey"
Press Enter