SalesOrder/CreatePurchaseOrder mapped action in the webservice endpoints do not fire

https://{myendpoint}.acumatica.com/entity/{myentity}/v1/SalesOrder/CreatePurchaseOrder

I am attempting to invoke the webservice endpoint here (created by me): 

I have created another request where the mapped action is the createPurchaseOrder@replenishment to attempt that as well:

 

I want to simulate the conditions of this button

here is the postman request (screenshot 3)

I am unable to find anything anywhere to understand what parameters may be required, and the reponse is downright confusing.

The error returns this- 
    "innerException": {
        "message": "An error has occurred.",
        "exceptionMessage": "Purchase Order '{0}' created.",


The ambition is to process the order below, but i have zero feedback from the api’s themselves on missing required parameters or other oversights.

Any assistance would be appreciated,

Chris