Add some basic markers to a map in mapbox via mapbox gl js
<p>I have a map styled with mapbox studio, however I'm having difficulty adding even a basic marker to it, however text is appearing where the marker should be which suggests that the marker would be there.</p>

<p>So here's the code with that map style:</p>

<pre><code>mapboxgl.accessToken = 'pk.eyJ1Ijoic21pY2tpZSIsImEiOiJjaWtiM2JkdW0wMDJudnRseTY0NWdrbjFnIn0.WxGYL18BJjWUiNIu-r3MSA';
var map = new mapboxgl.Map({
    container: 'map',
    style: "mapbox://styles/smickie/cikb3fhvi0063cekqns0pk1f1",
    center: [-30.50, 40],
    zoom: 2,
    interactive: false
});
</code></pre>

<p>And here some markers being added from an example in the api:</p>

<pre><code>map.on('style.load', function () {

    map.addSource("markers", {
        "type": "geojson",
        "data": {
            "type": "FeatureCollection",
            "features": [{
                "type": "Feature",
                "geometry": {
                    "type": "Point",
                    "coordinates": [-77.03238901390978, 38.913188059745586]
                },
                "properties": {
                    "title": "Mapbox DC",
                    "marker-symbol": "monument"
                }
            }, {
                "type": "Feature",
                "geometry": {
                    "type": "Point",
                    "coordinates": [-122.414, 37.776]
                },
                "properties": {
                    "title": "Mapbox SF",
                    "marker-color": "#ff00ff"
                }
            }]
        }
    });

    map.addLayer({
        "id": "markers",
        "type": "symbol",
        "source": "markers",
        "layout": {
            "icon-image": "{marker-symbol}-15",
            "text-field": "{title}",
            "text-font": ["Open Sans Semibold", "Arial Unicode MS Bold"],
            "text-offset": [0, 0.6],
            "text-anchor": "top"
        }
    });
});
</code></pre>

<p>However only the text and not the icons appear.</p>

<p>Question is: how would I add just a normal basic colored marker to this map, not even one of the special icon ones?</p>

<p>Thanks.</p>