How to initialize bar code scanner on App startup.

I have a requirement for my xamarin cross platform application that as soon as app start up .QR Scanner set in to read the code. on completing scanning a beep will be ring up.and app again ready for next scanning how can i get this done. what i have done is on button click scanner start, its read code, then i have to press button again to start it again.

public HomePage()
{
Button scanBtn = new Button
{
Text = "Scan Barcode",
HorizontalOptions = LayoutOptions.FillAndExpand,
};
scanBtn.Clicked += async (sender, args) =>
{
var scanResult = await Acr.BarCodes.BarCodes.Instance.Read();
if (!scanResult.Success)
{
await this.DisplayAlert("Alert ! ", "Sorry ! \n Failed to read the Barcode !", "OK");
}
else
{
var endpoint = new EndpointAddress("http://192.168.15.33/SMS/WebServices/SMSService.svc");
var binding = new BasicHttpBinding
{
Name = "basicHttpBinding",
MaxBufferSize = 2147483647,
MaxReceivedMessageSize = 2147483647
};
TimeSpan timeout = new TimeSpan(0, 0, 30);
binding.SendTimeout = timeout;
binding.OpenTimeout = timeout;
binding.ReceiveTimeout = timeout;
_client = new SMSServiceClient(binding, endpoint);
_client.ValidateStudentAsync("123-admin");
_client.ValidateStudentCompleted += _client_ValidateStudentCompleted; ;
// await this.DisplayAlert("Scan Successful !", String.Format("Barcode Format : {0} \n Barcode Value : {1}", scanResult.Format, scanResult.Code), "OK");
}
};
Content = new StackLayout
{
Children = {
scanBtn
}
};
}