example.html

<html lang="en">

<head>
    <meta charset="utf-8">
    <script src="https://cesium.com/downloads/cesiumjs/releases/1.92/Build/Cesium/Cesium.js"></script>
    <link href="https://cesium.com/downloads/cesiumjs/releases/1.92/Build/Cesium/Widgets/widgets.css" rel="stylesheet">
</head>

<body>
    <div id="cesiumContainer" style="width: 100%; height:100%"></div>
    <script>
        // Cesium.Ion.defaultAccessToken = '<Your Access Token Here>';
        var viewer = new Cesium.Viewer('cesiumContainer', {
            baseLayerPicker: true,
            vrButton: true,
            geocoder: false,
            navigationHelpButton: false,
            selectionIndicator: false,
            shadows: true,
            timeline: true,
            sceneModePicker: true,
        });
        var tileset = viewer.scene.primitives.add(new Cesium.Cesium3DTileset({
            url: "tileset/tileset.json"
        }))

        Cesium.when(tileset.readyPromise).then(function (tileset) {
            viewer.flyTo(tileset)
            var heightOffset = -20.0;
            var boundingSphere = tileset.boundingSphere;
            var cartographic = Cesium.Cartographic.fromCartesian(boundingSphere.center);
            var surface = Cesium.Cartesian3.fromRadians(cartographic.longitude, cartographic.latitude, 0.0);
            var offset = Cesium.Cartesian3.fromRadians(cartographic.longitude, cartographic.latitude,
                heightOffset);
            var translation = Cesium.Cartesian3.subtract(offset, surface, new Cesium.Cartesian3());
            tileset.modelMatrix = Cesium.Matrix4.fromTranslation(translation);
        })
    </script>
</body>

</html>