Do you want the message content as well as a link back, or just part of the message content and a link saying something like [...more]?
The first could be done with something like
PHP Code:
$post[message] = $message."
[url=".$vboptions['bburl']."/calendar.php?".$session[sessionurl]."do=getinfo&e=".$eventid."&day=".date("Y-n-j")."&c=".$calendarid."]Click here for full info[/url]";