1) After you've created the Form block in your app and added all the fields to the form you need to go to the "Developer" section of your app.

2) There you will see all the web services that are available in your app. Please select the web service that belongs to your form.
3) Next click the "Input Parameter" tab
4) Than select the "Body" tab.
5) Mark all the parameters as required.
6) Click "Save"