Your question is not completely clear to me. Do you want to trigger a microflow when a user typed something in a textbox that was empty before? Perhaps you can use an on change event here. This will trigger a microflow when you changed a field when leaving the widget (see https://docs.mendix.com/refguide/on-click-event for more info).
In this microflow you can check the whether the previous value was empty. To do this, you can for example retrieve the object from the db (which contains the old value of the entity). Same counts for dropdowns and other input widgets.