default session name in php is "PHPSESSID", and in this example youre using "session". You have to be very careful at determining the session name.
If you have changed session name to "session", then you can do the following:
$id= session_id(); // This is the id number (key) of the sess
echo "<form method=post action=\"whatever.php?session=$id\">";
otherwise you substitue "session" with the default name "PHPSESSID".
If your session was url pending then the session name AND the session number should've been auotomatically displayed in the URL.