Use this in your .js file

Notification.requestPermission();

Create a button for "Enable Notification" in .html file

<button id="enable">Enable notifications</button>

When pressing this button app requests notification for the app (.js file)

    function askNotificationPermission() {
  // Asking for permissions
  function handlePermission(permission) {
    // button set for shown or hidden, depending on the user's selection
    if(Notification.permission === 'denied' || Notification.permission === 'default') {
      notificationBtn.style.display = 'block';
    } else {
      notificationBtn.style.display = 'none';
    }
  }

  // Check if the browser actually supports notifications
  if (!('Notification' in window)) {
    console.log("This browser does not support notifications.");
  } else {
    if(checkNotificationPromise()) {
      Notification.requestPermission()
      .then((permission) => {
        handlePermission(permission);
      })
    } else {
      Notification.requestPermission(function(permission) {
        handlePermission(permission);
      });
    }
  }
}