There is a VIN pin for the motor, but I don’t see a VIN pin on the other side of the board (near DIR, PWM). I have motor VIN and GND connected to a 12v power supply and OUTA and OUTB connected to a brushed DC motor. Then I have GND on the other side, DIR, and PWM connected to an Arduino. I was able to make this work at one time, but I don’t remember how I connected the board to Arduino power.
To clarify, the G2 High-Power Motor Driver 18v17 board uses VIN to power the motor and logic on the board. If you are trying to power the Arduino from your motor voltage, you could use the VM pin (as long as you are using an appropriate voltage). The VM pin is the motor power supply voltage after reverse-voltage protection, so you can use it to power other components in your system, but it should not be used for powering high-current devices (such as another motor).