You will need to either create a session value https://www.ironistic.com/using-php-sessions-in-wordpress/ or pass use a query string value ?variable=value
You will need to pass the post ID of the post that the custom field is saved to to the thank you page using one of these two methods.
Then you can use this value to get the custom fields from that post id
$value = get_field('campaign_url', $post_id);