Example of project settings for Extended QGIS Web Client. Features: search with project data with WMS standard using search panels and external services (MapBox) for geocoding, reverse geocoding and getting elevation data.

eu_demo.json

{
	"search": [{
		"title": "Places",
		"useWmsRequest": true,
		"queryLayer": "pop_places",
		"formItems": [{
			"xtype": "textfield",
			"name": "name",
			"fieldLabel": "Name",
			"allowBlank": true,
			"blankText": "Please enter a name",
			"filterOp": "ILIKE"
		}, {
			"xtype": "textfield",
			"name": "iso_cc",
			"fieldLabel": "Country code",
			"allowBlank": true,
			"blankText": "Enter Country code (2 letters)",
			"maxLength": 2,
			"minLength": 2,
			"filterOp": "ILIKE"
		}, {
			"xtype": "combo",
			"name": "type",
			"store": [
				"National Capital",
				"Provincial Capital",
				"Large City",
				"Town"
			],
			"fieldLabel": "Type",
			"emptyText": "",
			"typeAhead": false,
			"forceSelection": true,
			"selectOnFocus": true,
			"filterOp": "="
		}],
		"gridColumns": [{
			"header": "Name",
			"dataIndex": "name",
			"menuDisabled": true,
			"renderer": "customURLFormatter"
		}, {
			"header": "Country code",
			"dataIndex": "iso_cc",
			"menuDisabled": true
		}],
		"selectionLayer": "pop_places",
		"selectionZoom": 12,
		"doZoomToExtent": false
	}],
	"geoCode": {
		"zoom": 19,
		"layers": "",
		"country": "AT",
		"provider": "mapbox"
	},
	"locationServices": [{
		"name": "elevation",
		"key": "your MapBox API access token",
		"provider": "mapbox"
	}, {
		"name": "address",
		"key": "your MapBox API access token",
		"provider": "mapbox"
	}]
}

Every part (search, geocode, locationServices) is optional, you don't need to have them all.

geocode.layers = MapBox type definition, can be address, place, poi,..., blank string for all types
geocode.country = 2 letter country code, can contain more values with comma separated. Blank string for worldwide search