Ofcourse you get an activation key. That is actually what you;re buying, as making a USB W10 drive is as easy as going to https://www.microsoft.com/en-us/software-download/windows10/
I looked up one of the sales on Amazon for a Windows 10 Home USB and in the answered questions section it says the key comes on a card with the USB.
Do you have a link to the one you are considering?