Hi,
if you first upgraded then you do not need to input an activation key. When the upgrade set up files checks your current operating systems status regarding if it's activated and genuine it checks an 'activation database' something recently created by Microsoft. If the current operating system is activated and genuine then a windows 10 licence is issued and stored in the same location.
This means that when you clean install you do not need to input an activation key. May I ask which key you are using?