[horde] Passing authentication info to horde?

[Demi]

Hello,

It's my 3rd day on this problem and I am just not getting this. Hope
someone will take a look. I am sure it's something easy.

My users are already authenticated via my own login script when they
enter the horde framework. Which hook do I use to pass my login info
to the horde framework.

I have sessions like following for username, password, last login and
can create anything else Horde might need.
$_SESSION['auth']['user']

if they are not authenticated then they should be send to my login
page via the Horde option: "If this is not false, it is assumed to be
the URL of an alternate login screen which will be used in place of
horde's default login screen"

How do I let horde know? I am not asking about imp I do mean login in
to the Horde framework.

Thank you

Demi