U can do it by:

Btn.ledOn+= ()
{
int a = 1;
byte aa = Convert.ToByte(a);
myConnection.thisSocket.OutputStream.WriteByte(Convert.ToByte(a)); // send 1 to arduino
}

Btn.ledOff+=
{
int b = 0;
byte bb = Convert.ToByte(b);
myConnection.thisSocket.OutputStream.WriteByte(Convert.ToByte(b)); // send 0 to arduino
}

then in arduino code u can read it by:

while(Esp_BT.available())
{
int received =Esp_BT.read();
Serial.println(received);

if(received ==0) //
{
Serial.println(“Disconnect”);
digitalWrite(led_b,LOW);
}
if(received==1) //100
{
Serial.println(“LED is ON !”);
digitalWrite(led_b,HIGH);
}

}

Is there any Bluetooth plugin required by Xamarin?