Setting focus to a specific field in an external program

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How do I set focus to a specific field in an external program? Our application is middleware that sends data to a different application through a port, but we then switch the screen focus to the other app. This is successful. What is unsucessful is setting the cursor to a specific field. The following is one of our efforts:

if (hChldAft != IntPtr.Zero)
{
    hNxtChld = WindowFuncs.FindWindowEx(hNxtChld, hChldAft, WindowFuncs.fesmChild3, "");
    StringBuilder ipSB = new StringBuilder("");                                 
    //WindowFuncs.SendMessage(h, WindowFuncs.WM_NEXTDLGCTL, hNxtChld, WindowFuncs.TRUE);
    WindowFuncs.PostMessage(h, WindowFuncs.WM_NEXTDLGCTL, hNxtChld, WindowFuncs.TRUE);
    WindowFuncs.SetFocus(hNxtChld);
    //WindowFuncs.SetFocus((IntPtr)264424);
    focusPlaced = true;
}

Thanks!