creating PO invoice using RESTful API

SOLVED

How do I create a PO invoice via the RESTful API? 
There are only GET methods

when trying to overcome this by creating an invoice using AP/APInvoiceBatches
in the body of the request in the field "SourceApplication" I've given the value "PO"

this isn't good enough for it doesn't create the PO invoice itself. 
please advise..