Edit in JSFiddle

var viewer = null

function initMap() {
  viewer = iv.getViewer();
  viewer.getMap().addControl(new Geoportal.Control.LayerSwitcher());
}

iv = Geoportal.load(
  'viewerDiv', ['jhyvi0fgmnuxvfv0zjzorvdn'], { // Positionnement par coordonnées
    lon: 2.294,
    lat: 48.858
  },
  13, {
    onView: initMap,
    layers: ['ORTHOIMAGERY.ORTHOPHOTOS'],
    overlays: {
      "wms": [{
        name: "Carte Géologique",
        url: "https://geoservices.brgm.fr/geologie",
        options: {
          params: {
            layers: "SCAN_D_GEOL50"
          }
        }
      }]
    }
  }
);
#viewerDiv {
  width: 100%;
  height: 400px;
}
<div id="viewerDiv"></div>

External resources loaded into this fiddle: