void setup() {
  Serial.begin(115200); // wake up serial at a specific rate 
  Serial1.begin(115200);// wake up serial1 at the same rate  (check that this is the rate your bluetooth module wants)
}

void loop() {
  if (Serial.available()) {      // If anything comes in Serial USB,
    Serial1.write(Serial.read());   // read it and send it out Serial1 (PA10/11)
  }

  if (Serial1.available()) {     // If anything comes in Serial1  (PA10/11)
    Serial.write(Serial1.read());   // read it and send it out Serial USB
  }
}