If you'd like to make the some of the fields in the Form (beta) block to be required, please follow these steps:

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