This overview makes use of the sample code that you can learn about on this post. This overview shows the flow from employee procurement portal through punchOut to supplier, return to requisition, purchase order and sending the purchase or to the supplier.
It’s worth noting the URL that will be used for sending the purchase order should be setup in the Order status check url for the external catalog. If the vendor that is associated with the purchase order isn’t related to an external catalog then the send won’t find the URL to submit the purchase order for that supplier.
Refer to this post for an overview of the cXML Purchase Order Send.