What payment method are you using? The default Payments Module or WooCommerce integration? and if you are using Payments Module then what Payment Gateway are you using (Stripe, PayPal or Bank Transfer)?

Ok, so if i understand correctly, then after a successful payment, you would like to select to which page PayPal should redirect the user?

If so then right now i am afraid the only option to do that is to open a file wpadverts-paypal-standard/wpadverts-paypal-standard.php go to line about 233 and change the line

$return_url = add_query_arg( array('adverts-notify-id'=>$data["payment_id"]), get_permalink( $data["page_id" ]));

to your selected URL for example

$return_url = "https://example.com/my/success/page/";

Note that the changes you will make in this file will be overwritten on WPAdverts PayPal update.

That being said in the next release i will add some filter or option which will allow changing the success page without modifying original files.