Add following code in your theme’s (child theme’s if any) functions.php (instead of second part of above code)

add_filter( 'fep_form_fields', function( $fields ){
	if( isset( $fields['message_title'] ) ){
		$fields['message_title']['label'] = __( 'Subject/Order number', 'front-end-pm' );
                $fields['message_title']['placeholder'] = __( 'Subject/Order number', 'front-end-pm' );
	}
	return $fields;
});