Use this code example to force show form for message action.

add_filter('frm_get_run_success_action_args', 'show_form_success_action_args', 10, 3);
function show_form_success_action_args( $new_args, $args, $action ) {
	if ( 'message' === $new_args['conf_method'] ) {
		$new_args['form']->options['show_form'] = 1;
	}
	return $new_args;
}