What is your model Apple TV? Identify your Apple TV model - Apple Support
It wants to know the serial number of the Apple TV box
What a strange question. I have never heard about a TV asking for a serial number of an HDMI device.
and it does not shaw any serial number on box.
The serial number should be printed on the bottom of the device, albeit in tiny lettering (see link above). It would also be printed on the packaging box that it originally came in.
It will also have the model number there, letter A followed by four digits: A1218, A1378, A1427, A1469, A1625, or A1842.