salesOrdersForOrderHub API - Unable to add lines
salesOrdersForOrderHub API - POST- adding order lines returns "SourceTransactionSystem and SourceTransactionId are mandatory when creating a new order"
We are using SoapUI, we can use a POST to the salesOrdersForOrderHub API, to create a sales order with 1 line. This works fine. Order is created and is visible in the front end and we get the appropriate response payload back with headerId, SourceTransactionId, etc.
I now want to add a line to this sales order that was created. I am using the same API with a POST. I have added a header on this update call with upsert-mode with a value of true. In the body of the payload, I am passing an OrderKey with attribute with the SourceTransactionSystem:SourceTransactionId and also SourceTransactionSystem:HeaderId. No matter which option I try, it seems to think I am trying to create a new sales order and returns "SourceTransactionSystem and SourceTransactionId are mandatory when creating a new order".