Cacher is the code snippet organizer for pro developers

<!-- turn off auto insert messages for the checkbox below only -->
<label data-bind="validationOptions: {insertMessages: false}">
  <input type="checkbox" data-bind="checked: wantsSpam" />
  I want spam
</label>
<div class="validationMessage" data-bind="validationMessage: wantsSpam"></div>

fieldName.extend({
    validation: [{
        validator: function (val) {
            if (val) {
                this.message = 'Wymagane jest zaznaczenie opcji "TAK"';
                return false;
            } else
                return true;
        }
    }]
});