APM HTTP Connector request and HTTP Headers

Hello, 

can someone share working solution for populating variables, used to send APM HTTP connector auth request, from HTTP headers?

User (API) sends credentials in HTTP headers X-LOGIN and X-TOKEN.

I tried to assign variables directly in per request policy, but this is not supported. I also tried to assign them using iRule:

when HTTP_REQUEST {
    set loginvalue [HTTP::header "X-LOGIN"]
    set tokenvalue [HTTP::header "X-TOKEN"]
    log local0. "Assigned variables are: LOGIN:$loginvalue TOKEN:$tokenvalue"
}

cURL request:

curl 1.2.3.4 --header "X-LOGIN: James" --header "X-TOKEN: Brown" --header "clientless-mode: 1"

Local traffic log shows correct assignment:

Rule /Common/headers_variables <HTTP_REQUEST>: Assigned variables are: LOGIN:James TOKEN:Brown

Direct using variables loginvalue and tokenvalue in HTTP connector request is not working, so I also tried to map them in PRP "Variable assign" block:
session.custom.login = Session Variable loginvalue 
session.custom.token= Session Variable tokenvalue 

But HTTP connector auth request
http://www.auth.com/api/v1/auth?login=%{session.custom.login}&token=%{session.custom.token}
is always empty, as seen from tcpdump capture:

[Full request URI: http://www.auth.com/api/v1/auth?login=&token=]

Any ideas please?