How set tab 2 in a xamarin forms Tabbed Page as the default tab on startup

You can access the enumerator of the TabbedPage's children and advance its position two times to get your "second tab". Assign that page as your CurrentPage:

public HomePage()
{
    InitializeComponent();
    var pages = Children.GetEnumerator();
    pages.MoveNext(); // First page
    pages.MoveNext(); // Second page
    CurrentPage = pages.Current;
}

public HomePage()
{
    InitializeComponent();
    CurrentPage = Children[2];
}