Sets the URL to app_url so that OneSignal WordPress Push Notifications open the app instead of phone browser. Disables Browser Notifications.

WP OneSignal Push To App.php

<?php
/*
  Plugin Name: WP OneSignal Push To App
  Description: Sets the URL to app_url so that OneSignal WordPress Push Notifications open the app instead of phone browser. Disables Browser Notifications.
  Version: 1.0
  Author: Con & Anna Schneider
  Author URI: https://conschneider.de
 */

// If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
	die;
}

add_filter('onesignal_send_notification', 'onesignal_send_notication_filter_push', 10, 4);

function onesignal_send_notication_filter_push($fields, $new_status, $old_status, $post)
{
    $fields['isAndroid'] = true;
    $fields['isIos'] = true;
    $fields['isAnyWeb'] = false;
    $fields['isChrome'] = false;
    $fields['url'] = $fields['app_url'];

    return $fields;
}