The seller claims it works for Mi Box 3 so it should work on Box S(dont know if they require the same power), but its the only seller that sells a USB powered adapter, everyone else sells regular power supplies. I dont know if the usb provides enought power to power my box.
I power a Mii Box S in my car with one of these cables. I use a normal car charger with an output of 2.1 Amps, the USB polarity is correct.
BTW: The cable is a DC barrel connector type B Sizes: 4 mm x 1,7 mm, also used by PSP