Addictivetips published a tutorial about how to find lost license keys and serial numbers for apps on Windows 10.

How to find lost license keys and serial numbers for apps on Windows 10

Paid software authenticates a user in one of two ways; a license key or a serial number that is given at the time of purchase, or by having the user sign in to an online account that was used to make a purchase. 

Both methods have their own advantage. Authentication through an online account means you only need a username and password to verify a purchase. If you forget either of them, they’re easy to recover.

How to find lost license keys and serial numbers for apps on Windows 10