See documentation here: https://docs.mendix.com/howto/extensibility/build-native-widget/
When you use the widget generator it will produce that file for you. If you made your own custom widget you could copy that file over from the generated widget to your own widget to use. You could also unzip some other widgets from the apstore. It is somewhere in the lib directory of the widget. It should be called LICENSE and should containt the apache license text.