Hi,
you will not able to redirect the user once the submission is completed because at this point already some content is sent to the browser by the theme and the redirects will not work.
What you can do is redirect the user when on a [adverts_add] / Preview page he will click the “Publish Listing” button.
For example, the code below will do that
add_action( "template_redirect", function() {
$action = adverts_request("_adverts_action");
$post_id = absint( adverts_request( "_post_id" ) );
if($action != "save" || $post_id === 0 ) {
return;
}
// Save action was executed you can do redirect below
wp_redirect( "https://example.com" );
} );
This will, of course, work best if the Ad is posted for free then you can just update its post_status with wp_update_post() function or if you have some plan to handle the payments on your own, as with this code the user will not see the default success page or the payment form.
Probably to see how it looks like it would be best to paste the code above in your theme functions.php file and post a new Ad from page with [adverts_add] shortcode.