CompleteUrl
This URL will be used by Swedbank Pay when a payment is Completed or Failed.
If your integration subscribes to the onPaid and possibly the onFailed
event, no redirect or use of the completeUrl will take place. But if you do
not have any event handler for the onPaymentCompleted event, the Swedbank Pay
JavaScript will perform an HTTP redirect in the top frame to the completeUrl.
You will still need to do a GET request on the payment resource to find the
final status (Completed or Failed).