title: PayPal Rest setting notfyURL when creating a billing plan
body: <p>Is it possible to specify the notify-url for a billing plan when creating it? I am using the PHP Rest SDK 1.7.4.</p>
  
  <p>I did read somewhere that it is possible to do this in the merchant preferences, I have tried this but get the response:</p>
  
  <p><code>"VALIDATION_ERROR","details":[{"field":"merchant_preferences.notify_url","issue":"Field is invalid in a request."}]</code></p>
  
  <p>Is the recommendation to do this out of date like most of the information about using PayPal?</p>
  
  <p>How can I get a cancellation of a billing agreement to notify a PayPal IPN handler (without setting the IPN address in the PayPal UI)? I don't want to use webhooks.</p>