Some APIs require payload encoded as multipart/form-data. To send multipart requests from Exporteo, you need to add a Content-Type header set to multipart/form-data; boundary=ExporteoFormBoundary.
You also need to insert the multipart boundaries to the output template like in the code snippet below. The example includes three parts: username, password, and an xml file named orderxml. Please make sure to preserve the empty lines below each Content-Disposition.