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

AX2012 R3

Refer to this post for an overview of the cXML Purchase Order Send.

https://organicax.com/2017/02/08/cxml-purchase-order-request-send-ax2012-r3-v1/

Cheers

Lachlan