MainActivity.cs

        private void M_navigationView_NavigationItemSelected(object sender, NavigationView.NavigationItemSelectedEventArgs e)
        {
            AddBadgeNotificationToMenuItem(e.MenuItem);

            //...
        }
        
        private void AddBadgeNotificationToMenuItem(IMenuItem menuItem)
        {
            TextView textviewBadgeNotif = (TextView)MenuItemCompat.GetActionView(m_navigationView.Menu.FindItem(menuItem.ItemId));

            //Red square with rounded corner and white text color
            textviewBadgeNotif.SetBackgroundResource(Resource.Drawable.left_menus_itemBadge);
            textviewBadgeNotif.SetTextColor(Color.White);
            textviewBadgeNotif.Gravity = GravityFlags.Center;

            //square size = 25 x 25 dp
            FrameLayout.LayoutParams lparams = (FrameLayout.LayoutParams)textviewBadgeNotif.LayoutParameters;
            lparams.Height = (int)TypedValue.ApplyDimension(ComplexUnitType.Dip, 25, Resources.DisplayMetrics);
            lparams.Width = (int)TypedValue.ApplyDimension(ComplexUnitType.Dip, 25, Resources.DisplayMetrics);
            textviewBadgeNotif.LayoutParameters = lparams;

            //Show or hide badge notification
            if (textviewBadgeNotif.Text != "")
            {
                textviewBadgeNotif.Text = "";
                textviewBadgeNotif.Visibility = ViewStates.Gone;
            }
            else
            {
                textviewBadgeNotif.Text = "1";
                textviewBadgeNotif.Visibility = ViewStates.Visible;
            }
        }