Simply add the FormConfirmPopupWebBlock web block into the footer of your app. It will detect value changes in all your input fields. If you want to monitor the value change of other kinds of UI elements, you can modify the code in the OnReady of the web block. Out of the box, the web block uses the Windows alert box for the alert. If you wish to use OutSytemsUI Message, you can also modify the ShowPopup client action