string path = @"D:\software\unitypack\2019.4.38f1c1\Editor\Unity.exe";
ProcessStartInfo psi = new ProcessStartInfo(path);
Process p = new Process();
p.StartInfo = psi;
p.Start();

private void buttonClick() {
  Thread th = new Thread(Test);
  th.IsBackground = true;
  th.Start("123");
}
 
private void Test(object s) {
  for (int i = 0; i < 10000; i++) {
    Console.WriteLine(i);
  }
}