Date: 2017-11-13
https://github.com/qgis/QGIS/blob/master/python/plugins/MetaSearch/dialogs/maindialog.py#L759-L792
In QGIS Python Console
ows_provider = QgsProviderRegistry.instance().createSelectionWidget('wms')
ows_provider.addRasterLayer.connect(iface.addRasterLayer)
ows_provider.setModal(False)
ows_provider.show()
ows_provider.btnConnect_clicked()