Accessing Custom Request Header fields inserted from Back end Application

Try this:

when HTTP_REQUEST {
    log local0. "Request URI = [HTTP::uri]"
    foreach x [HTTP::header names] {
        log local0. "Request header($x) = [HTTP::header $x]"
    }
}
when HTTP_RESPONSE {
    log local0. "Response status = [HTTP::status]"
    foreach x [HTTP::header names] {
        log local0. "Response header($x) = [HTTP::header $x]"
    }
}