Citation on the FHIR specification website.

The Citation Resource enables reference to any knowledge artifact for purposes of identification and attribution. The Citation Resource supports existing reference structures and developing publication practices such as versioning, expressing complex contributorship roles, and referencing computable resources.

The body should be a valid FHIR object for Citation.

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 Citation.