Installing a relay is the best answer. BUT from the diagrams and images that I see no diodes are installed in parallel with the coil of the relay. This is required to kill the back EMF ( a high voltage that is developed when current is stopped through the coil by releasing the start button) to avoid damage to electronic devices. Connect the cathode to the positive terminal of relay coil and anode to the negative terminal.
in case you are not familiar with term "B+", it came from the tube type devices where tubes required A and B voltages to operate.
I have not heard of "B-" as this would mean a voltage below ground.