How to refer to subject Patient using Identifier in Encounter

encounter-subject-identifier.json

{
  "resourceType": "Encounter",
  "id": "emerg",
  "text": {
    "status": "generated",
    "div": "\u003cdiv xmlns\u003d\"http://www.w3.org/1999/xhtml\"\u003eEmergency visit that escalated into inpatient patient @example\u003c/div\u003e"
  },
  "status": "in-progress",
  "statusHistory": [
    {
      "status": "arrived",
      "period": {
        "start": "2017-02-01T07:15:00+10:00",
        "end": "2017-02-01T07:35:00+10:00"
      }
    },
    {
      "status": "triaged",
      "period": {
        "start": "2017-02-01T07:35:00+10:00",
        "end": "2017-02-01T08:45:00+10:00"
      }
    },
    {
      "status": "in-progress",
      "period": {
        "start": "2017-02-01T08:45:00+10:00",
        "end": "2017-02-01T12:15:00+10:00"
      }
    },
    {
      "status": "onleave",
      "period": {
        "start": "2017-02-01T12:15:00+10:00",
        "end": "2017-02-01T12:45:00+10:00"
      }
    },
    {
      "status": "in-progress",
      "period": {
        "start": "2017-02-01T12:45:00+10:00"
      }
    }
  ],
  "class": {
    "system": "http://terminology.hl7.org/CodeSystem/v3-ActCode",
    "code": "IMP",
    "display": "inpatient encounter"
  },
  "classHistory": [
    {
      "class": {
        "system": "http://terminology.hl7.org/CodeSystem/v3-ActCode",
        "code": "EMER",
        "display": "emergency"
      },
      "period": {
        "start": "2017-02-01T07:15:00+10:00",
        "end": "2017-02-01T09:27:00+10:00"
      }
    },
    {
      "class": {
        "system": "http://terminology.hl7.org/CodeSystem/v3-ActCode",
        "code": "IMP",
        "display": "inpatient encounter"
      },
      "period": {
        "start": "2017-02-01T09:27:00+10:00"
      }
    }
  ],
  "subject": {
    "identifier": {
      "system": "http://caringly.tech/hospital-patient-identifier",
      "value": "Patient?identifier=MLKCH-LA%CERNER%654321"
    }
  },
  "period": {
    "start": "2017-02-01T07:15:00+10:00"
  },
  "hospitalization": {
    "admitSource": {
      "coding": [
        {
          "system": "http://terminology.hl7.org/CodeSystem/admit-source",
          "code": "emd",
          "display": "From accident/emergency department"
        }
      ]
    }
  },
  "location": [
    {
      "location": {
        "display": "Emergency Waiting Room"
      },
      "status": "active",
      "period": {
        "start": "2017-02-01T07:15:00+10:00",
        "end": "2017-02-01T08:45:00+10:00"
      }
    },
    {
      "location": {
        "display": "Emergency"
      },
      "status": "active",
      "period": {
        "start": "2017-02-01T08:45:00+10:00",
        "end": "2017-02-01T09:27:00+10:00"
      }
    },
    {
      "location": {
        "display": "Ward 1, Room 42, Bed 1"
      },
      "status": "active",
      "period": {
        "start": "2017-02-01T09:27:00+10:00",
        "end": "2017-02-01T12:15:00+10:00"
      }
    },
    {
      "location": {
        "display": "Ward 1, Room 42, Bed 1"
      },
      "status": "reserved",
      "period": {
        "start": "2017-02-01T12:15:00+10:00",
        "end": "2017-02-01T12:45:00+10:00"
      }
    },
    {
      "location": {
        "display": "Ward 1, Room 42, Bed 1"
      },
      "status": "active",
      "period": {
        "start": "2017-02-01T12:45:00+10:00"
      }
    }
  ],
  "meta": {
    "tag": [
      {
        "system": "http://terminology.hl7.org/CodeSystem/v3-ActReason",
        "code": "HTEST",
        "display": "test health data"
      }
    ]
  }
}