Fuel Injection swap - power during cranking?

I'm fighting some demons in my 1978 Jeep CJ5. I'm reworking the 5.0 fuel injected engine wiring. The wiring harness instructions require a wire that is only hot during cranking to "kickstart" the ECM. It says that it shouldn't be hot all the time as it will cause a rev limiter condition. Since the Jeep never had fuel injection it doesn't have any wires that are only hot during cranking.

Does anybody have a suggestion as to how i can achieve this hot wire only during cranking?

Thanks in advance.
Drew