public partial class TabbedPage
{
    public BottomNavigationView? BottomNavigationView => _tabbedPageManager?.BottomNavigationView;

    // ...
}

static void MapBadgeText(ITabbedViewHandler handler, TabbedPage view)
{
    var bottomNavigationView = view.BottomNavigationView;
    if (bottomNavigationView == null) return;

    int pageIndex = // (get this somehow)
    IMenuItem? menuItem = bottomNavigationView.Menu.GetItem(pageIndex);

    if (string.IsNullOrEmpty(value))
    {
        bottomNavigationView.RemoveBadge(menuItem.ItemId);
    }
    else
    {
        BadgeDrawable badge = bottomNavigationView.GetOrCreateBadge(menuItem.ItemId);
        badge.SetVisible(true);
        if (int.TryParse(value, out int badgeValue))
        {
            badge.Number = badgeValue;
        }
        else
        {
            badge.Text = value;
        }
    }
}