code: public Option addItem(String text, T value) {
          if (!values.contains(value)) {
              Option option = buildOption(text, value);
              values.add(value);
              listbox.add(option);
              return option;
          }
          return null;
      }
docs: Add Value directly to combobox component
  
  @param text  - The text you want to labeled on the option item
  @param value - The value you want to pass through in this option