Where is the UR-RT4 Serial Number?

Hi. I purchased a used UR-RT4 from Sam Ash, here in the U.S. I’d like to register it on the My Steinberg portal, but I can’t find the unit’s serial number on the unit. Can you please help me locate it? The My Steinberg hardware registration data fields aren’t accepting any of the numbers that I enter from the box itself (no, I don’t have caps lock on; I’ve tried multiples times in case I was making clerical errors)

The unit was unboxed when I bought it. As a result, I don’t have a box with any of the typical Steinberg paperwork (product pamphlets and one-sheets) that accompany the hardware.

Thank you.