Hi Francesco,
Here is also the sample JSON for a WMTS service as a basemap.
{"mapOptions":{"extent":{"ymax":4017215.044000647,"xmax":-13072293.072430076,"xmin":-13076396.785386244,"spatialReference":{"wkid":102100},"ymin":4014191.004069035}},"exportOptions":{"outputSize":[859,633],"dpi":96},"baseMap":{"title":"ESRI_Imagery_World_2D","baseMapLayers":[{"type":"wmts","url":"http://tiles.arcgisonline.com/arcgis/rest/services/ESRI_Imagery_World_2D/MapServer/WMTS/1.0.0/WMTSCapabilities.xml","layer":"ESRI_Imagery_World_2D"}]},"operationalLayers":[{"id":"PoolPermits","title":"PoolPermits","visibleLayers":[0,1],"url":"http://sampleserver6.arcgisonline.com/arcgis/rest/services/PoolPermits/MapServer","maxScale":0,"minScale":0}],"layoutOptions":{"legendOptions":{"operationalLayers":[{"id":"PoolPermits"}]}}}