APM HTTP auth - POST

with APM, I need to build the following flow

- user logs on APM logon page, credentials are validated with LDAP auth
- those credentials are sent to an existing auth webserver, with a POST (HTTP auth)
- that auth webserver replies with the same username (as the one sent in the form) if authorized

Question: can we use session variable in the "Successful Logon detection match value" field, as we look for "specific string in response"? Anyone successfully implemented that?