Hello. I am a bit puzzled about how to edit a string in a textarea from inside a game.

string stringToEdit = "enter text...";

void OnGUI()
{        
    if(tKeyPressed)
    {
        GUI.TextArea(new Rect (120, 10, 200, 100), stringToEdit, 160);
    }
}

I can see the textarea, can click into it and move the curser in the text but I cannot edit the text. What did I forget?