Also, start one workflow manually from the workflow builder or SWUS. Before starting, fill the container manually. Then after the workflow has started, execute SAP_WAPI_READ_CONTAINER for the work item that was created by your test case. You will see the format in which the container values are. This should give you a good idea about how to give container elements as parameters for SAP_WAPI_START_WORKFLOW.
Kind regards,
Karri