Dynamic URL to reference custom PHP files

I’m not positive I understand the question, but if you’re just trying to get the site URL, you can use the get_site_url() WP function. So, like this:

$siteURL = get_site_url();
header("=Location: $siteURL" . "/myoriginalformpage/?success=", $success);

And you can change the parameters of get_site_url() to specify the path and to display ‘https’ or ‘http’.

But if you just need the current site’s url, use get_site_url();