The geoprocessing tool allows the user to run it as the Geoprocessing widget without filling out all parameters in ArcGIS Web AppBuilder in Portal for ArcGIS but not in ArcGIS Web AppBuilder, in ArcGIS Online. When the geoprocessing tool is published as a service to ArcGIS Server and has optional parameters, it is not allowed to run as the Geoprocessing widget in ArcGIS Web AppBuilder, in ArcGIS Online.