Well you should definitely try our Add and edit features sample to see how it can be done (see the code within Qt creator). This is first sample available within the AppStudio desktop when creating a new app under Samples Category.
Also I would suggest you to look at our QuickReport template application. It uses a feature service layer. This is a data collection app primarily used as a reporting tool to collect information both spatial and non-spatial and push it as a feature to the feature service layer. This works offline also. Plus supports ArcGIS OAuth2 based secured services and supports Points, Polylines and Polygons geometry. Please let me know if you need more information on this. Thanks
-Nakul