code: private HashMap<String, String> createTokenRequestParams(String grantCode) {
  
          HashMap<String, String> params = new HashMap<>();
  
          params.put("code", grantCode);
          params.put("client_id", preferences.clientId.get());
          params.put("grant_type", "authorization_code");
          params.put("redirect_uri", HTTP_LOCALHOST);
  
          return params;
      }
docs: Generate the params that will be used during the token request phase
  @param grantCode from the authorization phase
  @return Map with all the headers