what json structure does simple oauth expect?
It will depend if the backend can receive a JSON format. 

In my case, I am working with the Drupal 8 Module simple Oauth. and the The format for OAuth 2.0 Bearer tokens is actually described in a separate spec, RFC 6750. 

More concrete here


  The entity-body follows the encoding requirements of the
  "application/x-www-form-urlencoded" content-type as defined by HTML
  4.01 [W3C.REC-html401-19991224].


So, In my particular case, I will try to send a form-data from angular.