You don't need a license key to
download or
intsall Windows 10 on any system.
You only need a key to
activate the installation once it's installed. (and you don't necessarily have to activate, as others here have confirmed.)
You can grab the media creation tool directly from Microsoft here:
Installation Media Creation Tool
It can help you create a USB drive with the installer on it, or you can dump it to an ISO file so you can burn it to a DVD if you need that instead.
A license key from a prebuilt system (HP, Dell, whatever) is usually an OEM key, that is tied to the specific machine it came with (usually the motherboard). You can activate Windows 10 on the machine the key is from.. but you can't activate that key on another machine. (I've had 100% success with Windows 7 keys on the same machine working in Windows 10, even a clean install; Vista not so much...)