Add a Normal Navigation Flow, which will be the Ajax call (for this example named: ajaxFlow).
Then you need to go to the Form Component, then go to the Outline view.
Then go to the field (ajaxCall), the one you want to add the Behavior, then on the properties of the field, go to ajax, and then Check AJAX, and select the Behavior and select the flow (ajaxFlow).
So now, when the field ajaxCall is changed will execute the ajaxFlow.
So easy!..
Regards