You won't need the key to get the iso or even begin the install process. For most people, when installing Windows 10, you can just skip the key entry windows until the install is complete and you have access to the Windows 10 shell. From there, open the settings pane and look for some tab about "Activation". If you have properly reserved the upgrade for yourself, then the activation should be seamless, though I have yet to try it myself. You may need to log in to a Microsoft account to activate.
TL;DR - You don't need a key to get the ISO