Edit in JSFiddle

function go() {
    var map = L.map('map', {
        crs: L.geoportalCRS.EPSG2154,
    }).setView([48.845, 2.424], 8);
    var lyr = L.geoportalLayer.WMTS({
        layer: "ORTHOIMAGERY.ORTHOPHOTOS.BDORTHO.L93"
    });
    lyr.addTo(map); // ou map.addLayer(lyr);
}

Gp.Services.getConfig({
    apiKey: "jhyvi0fgmnuxvfv0zjzorvdn",
    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;
}