Microsoft stopped embedding serial numbers since end of Windows XP ( .. Yes, Windows XP has hidden serial number inside the disc content, we can retrieve it using command prompt) .. You may again fully scratch carefullt the entire label, since the product key is printed very small, else;
Use a previous product key: If you had previously activated Windows on the same computer, you might be able to use that product key again.
Contact the manufacturer or retailer: If your computer came with Windows pre-installed, you can contact the manufacturer or retailer and ask if they can provide you with a product key.