Open And Find Another Control of a Form

I have a Window Applicattion, i am opening another application Inside that, Using Below Code.

 ProcessStartInfo startInfo = new ProcessStartInfo();
        startInfo.FileName = @"C:\Data.exe";
            Process p = Process.Start(startInfo);
            Thread.Sleep(500); // Allow the process to open it's window
           SetParent(p.MainWindowHandle, this.Handle);
      

 

Here File Is Opening, now i want to Find TextBox Of that Exe file, and write Some Data, any help will be Highly appreciate.