code: public static String encodeForm(Parameter<String> query, Charset charset) {
          try {
              return URLEncoder.encode(query.name(), charset.name()) + "=" + URLEncoder.encode(query.value(),
                      charset.name());
          } catch (UnsupportedEncodingException e) {
              // should not happen
              throw new RequestsException(e);
          }
      }
docs: Encode key-value form parameter