Ldap query from ltm

First thing, if this is a web-based application, then you probably don't need the ACCESS::session create function. This is done automatically when the browser connects. What you should probably do instead is capture the username in the URI, assuming it's coming in the first request, from the ACCESS_SESSION_STARTED event. Something like this:

when ACCESS _SESSION _STARTED {
    set username [whatever function you use to extract value from URI]
    ACCESS::session data set session.custom.username $username
}