Get auth token after the login of twitter or facebook?

I am using the U3DXT framework to access iOS apis and I am be able to login with faceebok or twitter user accounts stored in the device, but I need to retrieve the access token to use in my backend, is any way to retrieve that access token ?

I have tried:

         username = service.account.username;
         Debug.Log ("Account name setted to: " + AccountUserName);
 
         if (service.account.credential != null) {
             string token = service.account.credential.oauthToken;
             Debug.Log ("The oauth token is: " + token);
         
         } else { 
             Debug.Log ("The oauth token is null");
             MyUtils.DissmissLoadingDialog ();
         }

And the output is:

 Account name setted to: snidercillo
 The oauth token is null