code: private String fetch() throws IOException {
          return this.token
              .method("POST")
              .header(
                  "Content-Type",
                  "application/x-www-form-urlencoded;charset=UTF-8"
              )
              .header(
                  "Authorization",
                  String.format(
                      "Basic %s", DatatypeConverter.printBase64Binary(
                          new Utf8String(
                              String.format("%s:%s", this.app, this.key)
                          ).asBytes()
                      )
                  )
              )
              .fetch().as(RestResponse.class)
              .assertStatus(HttpURLConnection.HTTP_OK)
              .as(JsonResponse.class)
              .json().readObject().getString(PsTwitter.ACCESS_TOKEN);
      }
docs: Retrieve Twitter access token.
  @return The Twitter access token
  @throws IOException If failed