Arduino Json create json object

  DynamicJsonBuffer jsonBuffer3;

JsonObject &akar = jsonBuffer3.createObject();

akar["acctype"] = AccType;

// akar["access"] = type;  

akar["user"] = username;

akar["uid"] = uid;

akar["lokasi"] = deviceHostname;

String mqttBuffer;

String cobacoba;

akar.printTo(cobacoba);

//Serial.print("[ INFO ] Serial Publish:");

Serial.println(cobacoba);

how to mqttClient.publish a variable string

 void bacaserial(){

     if (Serial.available() > 0)

  {

    bt = Serial.readString();    //get data

    char charBuf[bt.length() + 1];   //modify it

    bt.toCharArray(charBuf,bt.length() + 1);

    mqttClient.publish("TAG/pesan", charBuf); // send

    Serial.println(bt);

    bt="";

    Serial.flush();

   }  

 }