How

When you send the Immunization resource, you will assign it a logical id.
Reference the logical ID of a previously created Patient resource in the patient reference field of the Immunization resource.
A status must be included. See the FHIR R4 spec for the status value set.
A vaccineCode must be included. Using a SNOMED CT code is preferred.
Include any other information in the Immunization, using coding when possible.
POST https://<client>.fhir-api.sandbox.patientsknowbest.com/fhir/Immunization

Very basic Immunization resource example:

{
 "patient": {
  "reference": "Patient/f4bd895b-0dbf-49d2-8130-4cb52679972d"
 },
 "vaccineCode": {
  "coding": [
   {
    "code": "310551000000106",
    "system": "http://snomed.info/sct",
    "display": "Booster diphtheria tetanus and five component acellular pertussis, haemophilus influenzae type b, inactivated polio vaccination (procedure)"
   }
  ]
 },
 "resourceType": "Immunization",
 "status": "completed",
 "id": "A828B85E-42B2-3C9A-4B4F-E02AECEDB5B3",
 "occurrenceDateTime": "1991-09-07T12:23:00+01:00",
 "performer": [
  {
   "actor": {
    "display": "Mr PKB Test",
    "reference": "Practitioner/66ce7f63-fb10-4afe-91cf-b10a0424f3e0"
   }
  }
 ],
 "text": {
  "div": "<div xmlns=\"http://www.w3.org/1999/xhtml\">Booster diphtheria tetanus and five component acellular pertussis, haemophilus influenzae type b, inactivated polio vaccination (procedure)</div>",
  "status": "generated"
 }
}

Creating Immunizations

How