code: @Override
      public Token getAccessToken(Token requestToken, Verifier verifier) {
          OAuthRequest request
                  = new ProxyOAuthRequest(this.api.getAccessTokenVerb(),
                          this.api.getAccessTokenEndpoint(), this.proxyHost, this.proxyPort);
          String userpass = this.config.getApiKey() + ":" + this.config.getApiSecret();
          String basicAuth = "Basic " + javax.xml.bind.DatatypeConverter.printBase64Binary(userpass.getBytes());
          request.addHeader("Authorization", basicAuth);
          request.addBodyParameter("grant_type", "authorization_code");
          request.addBodyParameter("code", verifier.getValue());
          request.addBodyParameter("redirect_uri", config.getCallback());
          Response response = request.send();
          String body = response.getBody();
          JsonNode json = JsonHelper.getFirstNode(body);
          if (json != null) {
              return new Token((String) JsonHelper.get(json, "access_token"), "", body);
          } else {
              return null;
          }
  
      }
docs: Makes the call to request an access token for an authorized user.
  
  @param requestToken not used.
  @param verifier the code query parameter that Globus included in the
  redirect after the authorization call.
  
  @return the access token.