I haven’t actually done this, but my first impulse would be to try and find a hook in order to modify the behaviour of Grav or one of its plugins. Looking at form.php, I see the event onFormValidationProcessed with the comment “Custom field logic can go in here”. You can hook into that event from your theme’s PHP:
public function onFormValidationProcessed(\RocketTheme\Toolbox\Event\Event $e)
{
dump($e['form']->fields());
}
My first impulse would be to try to modify destination with some setter function from classes/Form.php. Even if that works, though, I’d very carefully test my code before I trust my data on an approach from random strangers that start their advice with “I haven’t actually done this …”