index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Google Maps Test</title>

    <style>
        #map {
            height: 400px;
            width: 100%;
        }
    </style>

</head>
<body>
<h3>Simple Marker</h3>
<div id="map"></div>
<script>
    function initMap() {
        //Location
        var mySingleLocation = {lat: 47.3769, lng: 8.5417}; //Zürich

        //Map Options
        var options = {
            zoom: 8,
            center: mySingleLocation
        };

        //Init Map
        var map = new google.maps.Map(document.getElementById('map'), options);

        //Add Marker
        var marker = new google.maps.Marker({
            position: mySingleLocation,
            map: map,
            icon: 'img/location_02.png'
        });

        //Init Info Window of Marker
        var infoWindow = new google.maps.InfoWindow({
            content: 'Hallo'
        });

        //Get Info Window of Marker
        marker.addListener('click', function(){
            infoWindow.open(map, marker);
        })
    }
</script>

<script src="https://maps.googleapis.com/maps/api/js?key=AIzaSyDsm8sjiU9JAfnxnFFO5Jq7MsgNoGcvT7w&callback=initMap"></script>
</body>
</html>