This is my code that does a similar thing, (and also reads the output and return code)
process.StartInfo.FileName = toolFilePath;
process.StartInfo.Arguments = parameters;
process.StartInfo.UseShellExecute = false; // needs to be false in order to redirect output
process.StartInfo.RedirectStandardOutput = true;
process.StartInfo.RedirectStandardError = true;
process.StartInfo.RedirectStandardInput = true; // redirect all 3, as it should be all 3 or none
process.StartInfo.WorkingDirectory = Path.GetDirectoryName(toolFilePath);
process.StartInfo.Domain = domain;
process.StartInfo.UserName = userName;
process.StartInfo.Password = decryptedPassword;
process.Start();
output = process.StandardOutput.ReadToEnd(); // read the output here...
process.WaitForExit(); // ...then wait for exit, as after exit, it can't read the output
returnCode = process.ExitCode;
process.Close(); // once we have read the exit code, can close the process