Use sidebar nav icons in modules and plugins
You were almost there with it. You need to specify a path to an SVG. It can be one of the ones that exists within Craft's icon directory.

So your nav item would look like this:

Event::on(
    Cp::class,
    Cp::EVENT_REGISTER_CP_NAV_ITEMS,
    function(RegisterCpNavItemsEvent $event) {
        $event-&gt;navItems[] = [
            'url' =&gt; 'contact-enquiries',
            'label' =&gt; 'Contact Enquiries',
            'icon' =&gt; '@app/icons/envelope.svg',
        ];
    }
);


And this is how it will show in the CP nav: