args is a dictionary with the following keys:
fill_dict- The fill dictionary. Key/value pairs.
fill_source- None, BrowserDatabag, PreviousFormState, RecipientDatabag, or QueryString.
This callback is called after the form has completed rendering but before it is filled allowing you to intercept and alter the values which are pre-filled on the form.
Note: In order for this callback to work properly, you must return the args dictionary with your modified form_definition.