Here ya go.. I made this pictorial for someone lastyear when they asked the same question..
Flip the seat over so it is upside down. Outlined in BLUE is the helmet hook. One on each side of the seat..
Next is a picture of the helmet attached to the seat..
Finally here is a picture of the helmet/seat locked onto the bike.
EDIT:: Ok now after writieng all of that I realize what your were asking
No it's not possible to have the helmet attached under the seat and still have it sit on top of the seat. If you want to do this I recomend a CargoNet.
I sometimes take my spare helmet with me with it attached to the side of the bike like it is in the pictures but only if I'm going a short distance here in town otherwise I use a cargo net.