Follow the proper procedure to activate your win 10, that is upgrade your win 7 or 8.1 to win 10 first. Your win 10 key will only generate after the upgrade. Also remember to login to your Microsoft account to register your win 10 key. After the above procedure, then you can clean install using the ISO created. No key required during the installation procedure. Login to your Microsoft account to have your windows 10 activated. Any issue your can call Microsoft.
Do not clean install without performing the upgrade first.