code: public @NotNull LinkArgs property(String key, Object value) {
      if (key == null) {
        throw new IllegalArgumentException("Key argument must not be null.");
      }
      getProperties().put(key, value);
      return this;
    }
docs: Custom properties that my be used by application-specific markup builders or processors.
  @param key Property key
  @param value Property value
  @return this