code: public FormEncodingBuilder add(String name, String value) {
          if (content.size() > 0) {
              content.writeByte('&');
          }
          HttpUrl.canonicalize(content, name, 0, name.length(),
                  HttpUrl.FORM_ENCODE_SET, false, false, true, true);
          content.writeByte('=');
          HttpUrl.canonicalize(content, value, 0, value.length(),
                  HttpUrl.FORM_ENCODE_SET, false, false, true, true);
          return this;
      }
docs: Add new key-value pair.