How to use APM external logon pages?

when RULE_INIT {
    must recognize attempts to login to application (e.g., CareLink) 
    set static::login_action "/EpicCareLink/common/epic_check.asp" 
    set static::uid_field "Account_ID" }

when HTTP_REQUEST {
    set savecreds false
    if {([HTTP::uri] eq $static::login_action) && ([HTTP::method] eq "POST")} {
        client is attempting application login, so we will save the username 
        set savecreds true 
        set uid "" 
        set clen [HTTP::header Content-Length] 
        set clen [expr {(($clen eq "") || ($clen > 10240)) ? 10240 : $clen}] 
        if {$clen > 0} { HTTP::collect $clen } 
        return
    }
}

when HTTP_REQUEST_DATA {
    foreach field [split [HTTP::payload] "&"] {
        foreach {n v} [split $field "="] {
            if {$n eq $static::uid_field} {
                set uid [URI::decode $v] 
            }
        }
        if {$uid ne ""} { break }
    }
    HTTP::release
}

when ACCESS_ACL_ALLOWED {
    if { $savecreds } {
        ACCESS::session data set session.custom.carelinkusername $uid
    }
}