REST API for Adding Request Items

We are looking to create new EDMCS requests & items based on an API subscription from a third-party application. I've been able to create an empty request through the requests endpoint (REST API for Oracle Fusion Cloud Enterprise Data Management - Create a new request), and I see additional links in the API response (e.g., addNode) but I can't seem to get these links to work, or find any request body example to understand the JSON payload to be sent in the request body.

I'm aware of the importFile endpoint to upload a request item file, but from what I can tell this only supports Excel spreadsheets and not any other file types.