YII2: How can I create a link/url in a Menu item to point to an external webpage instead of an action or view of my application

add the target like below through the template setting. The Options you have set in your code are the Html Options of the li element and not the link options.

'items' => [
       'label' => Yii::t('backend', 'External link'),
       'url' => 'http://google.com',
       'icon' => 'fa-list-alt',
       'template'=> '<a href="{url}" target="_blank">{label}</a>',

Suggestions above do not seem to be working (in my case), an alternative solution is:

'linkOptions' => ['target' => '_blank']

For example

    'url' => \Yii::$app->user->identity->getBlogLink(),
    'linkOptions' => ['target' => '_blank']  ,
    'label' => \Yii::t('app', 'Blog')

If you want to keep the icon in your menu:

'template'=> '<a href="{url}" target="_blank">{icon}{label}</a>'

As regards the class you must specify it in the options key:

    'label' => 'Debug', 
    'icon' => 'fa fa-dashboard', 
    'url' => ['/debug'],
    'options' => ['class' => 'special'],
    'template'=> '<a href="{url}" target="_blank">{icon}{label}</a>',

gives the following menu:

<li class="special">
    <a target="_blank" href="your_site_url_here/index.php?r=debug">
        <i class="fa fa-dashboard"></i>

You can add any url. For Example,

echo Menu::widget([
    'items' => [
        ['label' => 'Home', 'url' => ['http://www.google.com']],
        ['label' => 'About', 'url' => ['site/about']],
     ['label' => 'Contact', 'url' => ['site/contact']],
    'options' => [
                    'class' => 'navbar-nav nav',
                    'style'=>'font-size: 14px;',