How to use 'onchange' in Yii2 MaskedInput widget
<p>I have an Yii2 app with advanced template. There is 2 fields on my form. 1st: MaskedInput and 2nd textInput with readonly attribute.
So, I wanted to fill 2nd textInput automatically right after I have filled MaskedInput. For this I tried to use onchange. But I got following error: </p>

<blockquote>
<pre><code>Unknown Property – yii\base\UnknownPropertyException
Setting unknown property: yii\widgets\MaskedInput::onchange
</code></pre>
</blockquote>

<p>Here is my code:</p>

<pre><code>&lt;?php $form = ActiveForm::begin(); ?&gt;
&lt;div class="row"&gt;
    &lt;div class="col-xs-3"&gt;
        &lt;?= Html::label("Ser num")?&gt;
        &lt;?= MaskedInput::widget(['name'=&gt;'serNum',
            'mask'=&gt;'AA 9999999',
            'onchange'=&gt;'
                $.post("index.php?r=act/actid&amp;serNum='.'"+$(this).val(),function(data){
                $("select#ser-sernum").html(data);
        });
            '
        ])?&gt;
    &lt;/div&gt;
    &lt;div class="col-xs-3"&gt;
        &lt;?= $form-&gt;field($model, 'sernum')-&gt;textInput(['readonly'=&gt;true]) ?&gt;
    &lt;/div&gt;
    &lt;div class="col-xs-6"&gt;
        &lt;b id="actstatus"&gt;&lt;/b&gt;
    &lt;/div&gt;
&lt;/div&gt;
</code></pre>