code: public void setFor(final IsWidget target) {
      if (init) {
        return;
      }
      init = true;
      //
  
      final InputElement input = getInputElement(target.asWidget());
      if (input != null) {
        if (!input.hasAttribute("id")) {
          input.setId(DOM.createUniqueId());
        }
        getElement().setAttribute("for", input.getId());
      }
    }
docs: set widget to reference to.
  
  @param target reference field