If you are looking to use the cXML Purchase Order send then you will need to enable a parameter on the vendor which will default to the purchase order for it to be picked up. This parameter is assessed when the purchase order is confirmed.
Here is a quick overview
Refer to this post for an overview of the cXML Purchase Order Send.
Pingback: cXML Purchase Order Request Send AX2012 R3 V1 | OrganicAX·