A 4-digit code located on the earbud body or the eartip. You may need to remove the silicone tip to see it.
The 17-digit serial number will be located on the inside of the earcup. You may have to slightly lift the inner fabric to see the label.
Check the headband for a 4-digit code. If you are unable to locate one, remove the cushion to reveal the 17-digit serial number on the earcup. If your headphones use a rechargeable battery, remove the battery and look inside the battery compartment.