There are some "custom" settings that will allow you to do this. It's in the documentation, I think there are alternative configs that are not tied to the launch platform/pads.
Actually it's that simple. It's not a module that activates the countdown, just a saved multistage2 guidance. If you press P to activate the countdown and then close the scenario, Orbiter will save the current state of the multistage guidance. When you load the scenario again, it will resume the countdown/launch at the moment that you closed orbiter.