BluetoothSerial bluetoothSerial;

Serial.println("Initializing Bluetooth...");
if(!bluetoothSerial.begin("ESP32", true)) {
    Serial.println("Error initializing Bluetooth");
} else {
    //bluetoothSerial.enableSSP();
    bluetoothSerial.onConfirmRequest([](uint32_t num_val){
        Serial.println("BT Confirm Request");
    });
    bluetoothSerial.onData([](const uint8_t *buffer, size_t size){
        Serial.println("BT Data");
    });
    bluetoothSerial.onAuthComplete([](boolean success){
        Serial.println("BT AuthComplete");
    });
    bluetoothSerial.setPin("1234");
}

... in loop():
if(bluetoothSerial.available()) {
    Serial.write(bluetoothSerial.read());
}