Hi and
to the forum.
If your kit doesn't have a pedelec connector fitted, your harness should still have the short cable with the four thin wires for pedelec and reverse coming out of the multijunction block unit closest to the motor, as labeled in the top of this diagram:
But you may need to carefully trim back the cable sheath slightly to access the four individual wires.
Simply connect the Black and Blue wires
(using a piece of suitable two core wire to extend it) to a suitable switch that joins the two wires together to select reverse, and separates them again to return the motor to its normal
(forward) direction of rotation.
Make sure the joined wires
(and the unused Yellow and Red pedelec wires) are safely insulated to prevent the copper strands of wire from accidentally touching against any of the other wires.
Alan