The purchase order send makes use of a batch job so the user isn’t waiting when they confirmation the purchase order. The batch job will update the status in the purchase request form so you can review this for sending status and if there are errors returned from 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.
Here is a quick overview.
The code uses the .NET Framework HTTP objects to submit the purchase order to the supplier. This means that there isn’t a technical requirement beyond having the .NET Framework current on the machine running the AOS where the batch job will run.
Refer to this post for an overview of the cXML Purchase Order Send.