I’m assuming you want class ‘thickbox’ added to the link that appears in the infoWindow around the title, when you open a marker?
If so you can copy the code from get_title_link and modify it (it doesn’t accept ‘class’ as an argument unfortunately).
$link = ($poi->postid) ? $map->options->mashupLink : false;
echo ($link) ? “postid) . “'>$poi->title” : $poi->title;