We have a training class starting on August 12th that covers this: https://ibupdate.jsmpros.com/ . The two scenarios we cover are creating an absence and creating a worker. We POST from PeopleSoft to an Oracle HCM Cloud instance. I completely agree that it is challenging to figure out. Here is my recommendation:
Start with SoapUI or some other local REST client and make sure you can post to the HCM cloud using a simple, local client. Most of the Oracle documentation examples use CURL, but that might be a bit more challenging with a POST. The hardest part for me was trying to figure out what parameters are required, and which values to use. HCM Cloud REST documentation is here: https://docs.oracle.com/en/cloud/saas/human-resources/20a/farws/index.html
Most of the POST request messages may be created as Document-based messages, but I prefer unstructured messages with JSONObject. Otherwise, it is the exact same as any other REST POST from Integration Broker.