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();

   }  

 }