Composition on the FHIR specification website.
A set of healthcare-related information that is assembled together into a single logical package that provides a single coherent statement of meaning, establishes its own context and that has clinical attestation with regard to who is making the statement. A Composition defines the structure and narrative content necessary for a document. However, a Composition alone does not constitute a document. Rather, the Composition must be the first entry in a Bundle where Bundle.type=document, and any other resources referenced from Composition must be included as subsequent entries in the Bundle (for example Patient, Practitioner, Encounter, etc.).
The body should be a valid FHIR object for Composition.
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 Composition.