ChargeItem on the FHIR specification website.

The resource ChargeItem describes the provision of healthcare provider products for a certain patient, therefore referring not only to the product, but containing in addition details of the provision, like date, time, amounts and participating organizations and persons. Main Usage of the ChargeItem is to enable the billing process and internal cost allocation.

The body should be a valid FHIR object for ChargeItem.

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 ChargeItem.