ASP.NET

function getKeyCode (e_) {
  return parseInt(e_ ? e_.which : window.event.keyCode);
}
function srcEl(e_) {
  if (window.event)
    return window.event.srcElement;
  else
    return (e_ ? e_.target : null);
}
function onKeyUp(e) {
  var textbox = srcEl(e),
        keyCode = getKeyCode(e);

  var text = textbox.value;
    ....
    fill list box based on "text" string
    ....
}

textbox.onkeyup = onKeyUp;