dynamic auth url

I am trying to create a dynamic auth address


# grab ssoid
map $cookie_SSOID $ssoid_cookie {
default "";
~SSOID=(?P<ssoid>.+) $ssoid;
}


location /imaadmin/ {
proxy_cache off;
proxy_pass http://IMAAdmin;



auth_request /sso/validate?SSOID=$ssoid_cookie&a=imaadmin;


what I am trying to do is fill the variable ssoid_cookie with the
cookie value for SSOID in the request or make it blank

then when somebody tries to access /imaadmin make the auth request
/sso/validate?SSOID=$ssoid_cookie&a=imaadmin;

but i get this
GET /sso/validate%3FSSOID=$ssoid_cookie&a=imaadmin HTTP/1.0

alex