c # Win32 api messages

I need to make an application of c # that activates an ok button in another different application. I do not have the code of the other one to plicacion and the sendkey does not work to me. I wanted to do it with win32 api. As in this example but it does not work for me, I do not know how to identify the handlers or I think so. I do not know if I have to use spy ++ or as if you can help me, I thank you!

    int hwnd=0;
IntPtr hwndChild=IntPtr.Zero;

//Get a handle for the Calculator Application main window
hwnd=FindWindow(null,"Calculator");
if(hwnd == 0)
{
    if(MessageBox.Show("Couldn't find the calculator" + 
                       " application. Do you want to start it?", 
                       "TestWinAPI", 
                       MessageBoxButtons.YesNo)== DialogResult.Yes)
    {
        System.Diagnostics.Process.Start("Calc");
    }
}
else
{

    //Get a handle for the "1" button
    hwndChild = FindWindowEx((IntPtr)hwnd,IntPtr.Zero,"Button","1");

    //send BN_CLICKED message
    SendMessage((int)hwndChild,BN_CLICKED,0,IntPtr.Zero);

    //Get a handle for the "+" button
    hwndChild = FindWindowEx((IntPtr)hwnd,IntPtr.Zero,"Button","+");

    //send BN_CLICKED message
    SendMessage((int)hwndChild,BN_CLICKED,0,IntPtr.Zero);

    //Get a handle for the "2" button
    hwndChild = FindWindowEx((IntPtr)hwnd,IntPtr.Zero,"Button","2");

    //send BN_CLICKED message
    SendMessage((int)hwndChild,BN_CLICKED,0,IntPtr.Zero);

    //Get a handle for the "=" button
    hwndChild = FindWindowEx((IntPtr)hwnd,IntPtr.Zero,"Button","=");

    //send BN_CLICKED message
    SendMessage((int)hwndChild,BN_CLICKED,0,IntPtr.Zero);

}