If you are upgrading an existing install of windows 7 or above that is already activated, then it should not prompt you for a key. The key is generated after the upgrade and you shouldn't have to enter one.
If you're doing a fresh install from scratch, then you should be able to enter your current windows 7, 8, or 8.1 key during setup. But if you're going this route- make sure you have the latest build which just came out (1151), As they've only just added this functionality recently.
Download the latest media creation tool from here: https://www.microsoft.com/en-us/soft...load/windows10
If you are unsure of what your product key is, you can use a tool such as Produkey
to reveal it. And lastly, as you you probably already know, windows 10 is only free as an upgrade. If you do not already have a license for win7 or above, you cannot upgrade for free.