[horde] how to by pass login page of imp

[Patrick DUBAU]

Horde :  3.1.3
Imp   :  4.1.3
OS    : debian etch

Hi,

we want our users to launch horde/imp through  our portal (only one identification
needed), so i want to by pass the login page from imp. 
I thought of something like : 

 http://x.x.x.x/horde3/imp/redirect.php?login_username=firstname.name&secretkey=password

but i don't know how to handle the 2 variables in redirect.php 

Is there any solution ?
Thx