$url = \yii\helpers\Url::to(['ajax-series-search', 'records' => ($records == 'sales') ?1 :2]);

'seriesNumber' => [
        'attribute' => 'id',
        'label'     => 'Series no.',
        'value'     => function ($model) {
          return $model->seriesNumber;
        },
        'filter'    => Select2::widget([
          'model'         => $searchModel,
          'attribute'     => 'id',

          'initValueText' => $searchModel->seriesNumber, // set the initial display text
          'options'       => ['placeholder' => 'Search a series ...'],
          'pluginOptions' => [
            'allowClear'         => TRUE,
            'minimumInputLength' => 2,
            'language'           => [
              'errorLoading' => new JsExpression("function () { return 'Waiting for results...'; }"),
            ],
            'ajax'               => [
              'url'      => $url,
              'dataType' => 'json',
//              'data'     => new JsExpression('function(params) { return {q:params.term}; }')
            ],
//            'escapeMarkup' => new JsExpression('function (markup) { return markup; }'),
//            'templateResult' => new JsExpression('function(city) { return city.text; }'),
//            'templateSelection' => new JsExpression('function (city) { return city.text; }'),
          ],
        ])
      ],