If you haven't changed your hardware then you can install Win10, skip the key and the installation should then activate automatically afterwards once done. Mine did anyway. MS creates a hardware ID that's tied to your key. At least I don't personally need to type in my keys when I reinstall.
@PCEGamer If you're key is locked to your motherboard and you are changing out your board you can always contact Microsoft as well as long as you have the key for it because for my cousins system his key was locked to his board and when i contacted MS for him they enabled they key for his new system.