It is not the /u and /p which is to be used. When accessing from a remote machine it should specify the /ru and /rp and they should belong to a user with admin privilege. The command is as following:
startInfo.UseShellExecute = false;
startInfo.WorkingDirectory = @"C:\Windows\System32";
startInfo.FileName = @"C:\Windows\System32\schtasks.exe";
startInfo.Arguments = "/create /tn <taskname> /tr <theEXE> /ru <user> /rp <password> /sc daily ";
startInfo.RedirectStandardError = true;
startInfo.RedirectStandardOutput = true;
startInfo.CreateNoWindow = false;
startInfo.WindowStyle = ProcessWindowStyle.Hidden;
proc = Process.Start(startInfo);