add-marker-custom-script.js

const customMapId = 1;

/* An array with objects, each object represents a marker */
const customMarkerData = [
	{
		title : "Example Title",
		address: "Example Address",
		map_id: customMapId,
		lat: 41.973052569247955,
		lng: -120.2621297558594,
		description: "Description",
	}
];

jQuery(function($){
  	$(document.body).on('markersplaced.wpgmza', () => {
   		const map = WPGMZA.getMapByID(customMapId);
      	if(map){
         	for(let i in customMarkerData){
             	const data = customMarkerData[i];
				const instance = WPGMZA.Marker.createInstance(data);
				map.addMarker(instance);
            }
        }
  	});
});