set the default value of TextField inside alertDialog

i found the following sample code from Appcelerator's docs (http://docs.appcelerator.com/platform/latest/#!/api/Titanium.UI.AlertDialog-property-previewContext):

    var dialog = Ti.UI.createAlertDialog({
    title: 'Enter text',
    style: Ti.UI.iOS.AlertDialogStyle.PLAIN_TEXT_INPUT,
    buttonNames: ['OK']
  });
  dialog.addEventListener('click', function(e){
    Ti.API.info('e.text: ' + e.text);
  });
  dialog.show();

The problem is that i need to set a default value for the input text, but the documentation only shows the possibility to set a placeholder, but not a default value for it.