OperationDefinition on the FHIR specification website.

A formal computable definition of an operation (on the RESTful interface) or a named query (using the search interaction).

The body should be a valid FHIR object for OperationDefinition.

For example, if creating a Slot resource, here is a valid object:

{
  "resourceType": "Slot",
  "schedule": {
    "reference": "schedule/a"
  },
  "status": "free",
  "start": "2022-08-10T11:00:00Z",
  "end": "2022-08-10T12:00:00Z"
}

You can reference the FHIR documentation to determine the format for OperationDefinition.