CApture output of other application

One thing that might work is to copy the selected text to the clipboard by sending the keystroke combination Alt + EC (or whatever combination it uses) to your application.

However for this to work, your application window would need to have the input focus at the time.

The following console application uses this technique to read the currently selected text in a Notepad window which has to become the active window within 5 seconds of starting the application:

using System;
using System.Windows.Forms;
using System.Threading;

class Program
{
     [STAThread]
     static void Main()
     {        
         Console.WriteLine("You have 5 seconds to switch focus to Notepad");         
         Thread.Sleep(5000);
         SendKeys.SendWait("%EC");
         string text = Clipboard.GetText();
         Console.WriteLine(text + " is currently selected");
         Console.ReadKey();  
     }
}