The ‘action’ property of an Async Button is a binding expression that resolves to managed bean method.
If you have an empty ‘action’ property, double clicking the button in the view editor will automatically create a new method in your Page Bean and create an ‘action’ property that resolves to that new method.
You are free to do anything you like in that method, such as invoking a web service client. The page bean fields you’ve bound to your 5 input controls will be updated when the async button is clicked (the form data is posted).