Edit in JSFiddle

function go() {

  // Projection UTM 20N 
  var utm20 = new L.Proj.CRS('EPSG:4559',
    '+proj=utm +zone=20 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs');

  map = L.map("map").setView([16.239, -61.545], 12);

  var lyrOrtho = L.geoportalLayer.WMTS({
    layer: "ORTHOIMAGERY.ORTHOPHOTOS",
  });
  map.addLayer(lyrOrtho);
  var mousePosition = L.geoportalControl.MousePosition({
      displayAltitude : false,
      systems : [
      {
         crs : utm20,
         label : "UTM 20 Nord",
         type : "Metric"
       },
      {
         crs : L.CRS.Simple,
         label : "Lon, Lat",
         type : "Geographical"
       },
     ],
     units : ["DEC", "DMS", "M"]     
});
  map.addControl(mousePosition);

}

Gp.Services.getConfig({
  apiKey: "essentiels",
  onSuccess: go
});

var infoDiv = document.getElementById("info");
infoDiv.innerHTML = "<p> Extension Leaflet version " + Gp.leafletExtVersion + " (" + Gp.leafletExtDate + ")</p>";

<div id="map"></div>
<div id="info"></div>

#map {
  padding: 5px;
  width: 100%;
  height: 300px;
  box-shadow: 0 0 10px #999;
}

#info {
  padding: 5px;
  width: 100%;
  height: 20px;
  font-family: monospace;
}