Cross-client authorization

I'm attempting to do  cross-client authorization  for an app I'm writing. I've modified my requests to include the  audience  field in both the auth request and the token request. However, the token returned is missing the  server_code  field that I need.

The Google Sign-In library  supports  this feature. However, the app I'm working on needs to allow a single user to authorize multiple Google accounts, so I'm stuck using the Google Toolbox for Mac solution. If there are other iOS library that support this, let me know.

The token request parameters whose response I expect a  server_code  field in:

  audience 
 < 
 server client ID 
 >. 
 apps 
 . 
 googleusercontent 
 . 
 com
client_id 
 < 
 iOS app client ID 
 >. 
 apps 
 . 
 googleusercontent 
 . 
 com
client_secret 
 < 
 iOS app client secret 
 > 
 code 
 4 
 / 
 GrpT1WGXsz4ertF97fchDU0SsXYGNJfzeWrcJF6VH5c 
 grant_type  authorization_code
redirect_uri  urn 
 : 
 ietf 
 : 
 wg 
 : 
 oauth 
 : 
 2.0 
 : 
 oob
scope https 
 : 
 // mail.google.com/ 
 https://www.googleapis.com/auth/userinfo.email