THe code below I copied from MSDN with a bit of modification:

[DllImport("user32.dll", CharSet = CharSet.Unicode)]
public static extern IntPtr FindWindow(string lpClassName,string lpWindowName);
DllImport("User32")]
public static extern bool SetForegroundWindow(IntPtr hWnd);
int cnt = 0;
private void button1_Click(object sender, EventArgs e)
{
     IntPtr calculatorHandle = FindWindow("Notepad", "Untitled - Notepad");
      if (calculatorHandle == IntPtr.Zero)
      {
          MessageBox.Show("Calculator is not running.");
          return;
      }
      SetForegroundWindow(calculatorHandle);
      SendKeys.SendWait(cnt.ToString());
      SendKeys.SendWait("{ENTER}");
      cnt++;
      SendKeys.Flush();
      System.Threading.Thread.Sleep(1000);
}

The problem is the number sequence in Notepad is not continuously. The first click always results 0 (as expected). but from the second click, the result is unpredictable (but the sequence is still in order, e.g. 3, 4, 5, 10, 14, 15, ....)

If I click the button fast enough, I was able to get the result in continuous order (0,1,2,3,4,....) but sometimes it produces more than 2 same numbers (e.g. 0,1,2,3,3,3,4,5,6,6,6,7,8,9,...)