Which API can I use to get PO details based on PO number

Summary:

I need to get details of a purchase order using an API. The API available is:

/fscmRestApi/resources/11.13.18.05/purchaseOrders/{purchaseOrdersUniqID}

But it needs a po_header_id as a parameter or some unique ID, which I am not clear how to generate. Is there a way I can get PO details using the PO number via an API call?

API documentation states:

• purchaseOrdersUniqID(required): stringThis is the hash key of the attributes which make up the composite key for the Purchase Orders resource and used to uniquely identify an instance of Purchase Orders. The client should not generate the hash key value. Instead, the client should query on the Purchase Orders collection resource in order to navigate to a specific instance of Purchase Orders to get the hash key.