code to use windows default voice in talkback code

Question

below is the code am usng in a text to voice application. i need help so that the application uses the windows 10 default

talkback voice.

using System.Speech;
using System.Speech.Synthesis;

private void Speak()
{
speechSynthesizerObj = new SpeechSynthesizer();   
speechSynthesizerObj.Volume = 100;
speechSynthesizerObj.SpeakAsync(richTextBox1.Text);
}

any help will be greatly appreciated