AdverseEvent on the FHIR specification website.

Actual or potential/avoided event causing unintended physical injury resulting from or contributed to by medical care, a research study or other healthcare setting factors that requires additional monitoring, treatment, or hospitalization, or that results in death.

The body should be a valid FHIR object for AdverseEvent.

For example, if updating 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 AdverseEvent.