c# and external .exe integration

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C#

I want to write a C# program which will launch an .exe (this is an easy job, I know how to do it.) but now I want to add some more functionality to it. I want to take user inputs to my program and want to pass them to running .exe as a input.Also I want to click on a particular button available on the exe window. Important thing is, I want to do it programatically without user knowledge that we are actually running that exe in background.

word = textBox1.Text;
           Process myprocess = new Process();
           myprocess.StartInfo.FileName = @"C:\Users\Public\Desktop\Cambridge Advanced Learner's Dictionary - 3rd Edition.lnk";

           myprocess.Start();
           myprocess.WaitForInputIdle();
           Thread.Sleep(3000);
           SendKeys.Send(word);

what to do next?

[Edit]Code block added[/Edit]