title: How to add validation to knockout
body: <p>I have a field in the class:</p>
  
  <pre><code>self.address = ko.observable().extend({
      required: true,
      maxLength: {
          params: 1000,
          message: 'Long address'
      },
      validation: {
          validator: function(val, substring) {
              return /*...*/;
          },
          message: 'Address not found',
          params: ''
      }
  });
  </code></pre>
  
  <p>HTML fragment:</p>
  
  <pre><code>&lt;div class="form-additem-group-row__col form-additem-group-row__col_field"&gt;
      &lt;input type="text" id="addressInput" class="field field_type2 input--full" placeholder="Enter the address" data-bind="textInput: address, geoModuleInput: { mapObject: map, debugSuggestCont: '#currentSuggest', valueText: address, valueKey: addressKey, valueMetroName: metroName, valueMetroDuration: metroDuration, valueGeoCodeResult: geoCodeResult }"/&gt;
      &lt;div class="form-additem-group-row__error" data-bind="validationMessage: address"&gt;&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Address processing takes place in the module <code>geoModuleInput</code> and whether an address exists or not.</p>
  
  <p>How to call validation if the address does not exist?</p>