POST /as/token.oauth2 HTTP/1.1
 Host: as.example.com
 Authorization: Basic cnMwODpsb25nLXNlY3VyZS1yYW5kb20tc2VjcmV0
 Content-Type: application/x-www-form-urlencoded

 grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Atoken-exchange
 &resource=https%3A%2F%2Fbackend.example.com%2Fapi
 &subject_token=accVkjcJyb4BWCxGsndESCJQbdFMogUC5PbRDqceLTC
 &subject_token_type=
  urn%3Aietf%3Aparams%3Aoauth%3Atoken-type%3Aaccess_token