Here is what I would do.

If it has a resistor (sometimes hidden) tap into that.

If it does not have a resistor tap into the run circuit on the ignition switch. Check to see if it goes dead while cranking. If not then use that to power the system but be sure the power to the system & coil power off when the key is removed.

If it goes dead when cranking then add a relay or diode to the system to power the coil during cranking without allowing the voltage to feed back into the crank circuit while running. I have more relays than diodes but a diode is cleaner looking.