The Rest and xmlrpc are for sync communication i.e. you send a request and get back a response.

What you are asking about is async communication i.e. the server sends you a notification whenever there is an event (create on po/so here)

It is also not clear where you want to publish this event to, i.e. do you want to just send an emai?

You may need to write your own solution here and it is not very difficult. 1. Setup a workflow where you intercept all create events 2. From the workflows, publish the event and bind it to a call back. 3. From the callback, if your processing is simple such as sending an email, there is no problem. But, if you need to notify an end point that is on an external network, I suggest using the pub/sub model and publish this event to a queue such as mqseries (or an opensource alternative)

I am providing more information than needed, since it may be useful for other readers.