Is this the right way to connect a Brushless Motor Driver Output Signal to a CNC BreakoutBoard?
The output will be relative to the Control Signal ground (GND1).  This is probably the same ground you have for all the other outputs from the BOB.

You can check this by using a multimeter to measure between the alarm output and GND1, you should see approximately 0V or 5V depending on the alarm state.

Once verified by a multimeter, you should have no issues directly connecting this signal to the BOB using the 330ohm resistor as outlined.  Most BOB boards have this resistor built-in and the opto-isolated may or may not transition with two resistors in series (660 ohm).