Pitch bend is controlled by an RPN. Send MIDI CC #101 value 0 then CC #100 value 0 to start sending updates to the RPN, then CC 6 (MSB) value 0-12 to set the pitch bend amount in semitones, then CC 101 value 127, then CC 100 value 127 to stop sending updates to the RPN. The CC101/100:127 isn't strictly necessary since the Mother 32 doesn't appear to have any other RPNs, but it keeps you from accidentally sending CC#6 and modifying the bend amount.
If you use Ableton, I created a Max for Live device for the Mother 32 that includes pitch bend control.