How do I move a script variable to an event sheet?

// Import any other script files here, e.g.:
// import * as myModule from "./mymodule.js";

runOnStartup(async runtime =>
{
	// Code to run on the loading screen.
	// Note layouts, objects etc. are not yet available.
	
	runtime.addEventListener("beforeprojectstart", () => OnBeforeProjectStart(runtime));
});

async function OnBeforeProjectStart(runtime)
{
	// Code to run just before 'On start of layout' on
	// the first layout. Loading has finished and initial
	// instances are created and available to use here.
	document.addEventListener("mousedown", (e) => {e.preventDefault(); mousedown(e.clientX, e.clientY, e.button)});
	
	
	runtime.addEventListener("tick", () => Tick(runtime));
}

function mousedown(x, y, button){
	console.log(button);
	runtime.globalVars.mouseButtonNum = button;
}

function Tick(runtime)
{
	
}