Switched 12v source

Hi guys I am installing a Holley sniper efi with Holley hyperspark ignition system on my 1975 Plymouth scamp with a 360. Holley says I need to hook up a certain wire to a switched 12v source (hot with key in ON position, and hot while cranking) and I cannot use the positive of the ignition coil they claim. I am really having trouble finding or figuring out where to go. I’m no electrician by any means. Any and all help will be greatly appreciated. Thanks so much

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