function insertAtCursor(myField, myValue) {
  //IE support
  if (document.selection) {
    myField.focus();
    sel = document.selection.createRange();
    sel.text = myValue;
  }
  //MOZILLA/NETSCAPE support
  else if (myField.selectionStart || myField.selectionStart == '0') {
    var startPos = myField.selectionStart;
    var endPos = myField.selectionEnd;
    myField.value = myField.value.substring(0, startPos)
                  + myValue 
                  + myField.value.substring(endPos, myField.value.length);
  } else {
    myField.value += myValue;
  }
}

// calling the function

insertAtCursor(document.formName.fieldName, 'this value');