By "event script section" do you mean the "Signal Script"? Or the PLC script? I'm not an expert (yet) on the Lua side of Mach4, but my understanding of the way things are arranged in the wx4 screen set (but NOT the wxMach screen set) is this:
PLC script - this runs at a periodic interval (default is 50ms). You could place the code there, but you would have to check for a change in state of the input signal. Do-able but more work and not as efficient as the following methods as this code would run even if input signal hasn't changed states. And there would be a variable delay (from 0ms to the "PLC Interval" defined in the top level of the screen set) in responding to the input signal. In your case that would not be an issue as all you are doing is enabling Mach4.
Signal script - this script, in the wx4 screen set, is just the dispatcher for Mach4 signal events. Every event in Mach4 runs this script, which simply checks to see if there is a script associated with that event (in the "signal library", which is just an array of functions indexed by event number) and if so, it runs the script. Again, you could place the code here, but that doesn't fit with the overall design of the event flow in the wx4 screen set.
Screen Load Script - This is where the "signal library" is defined, and where the screen set places code for its event handlers. The screen load script is run once at start up. Part of what is does is initialize the SigLib[] array with code to handle the various events. SigLib[] is then used by the signal script to process events. This code is only run when its associated event is triggered.