wordpress plugin add custom menu

//add custom menu to backend / admin dashboard

add_action('admin_menu', 'register_my_custom_menu_page');

function register_my_custom_menu_page()
{
    add_menu_page(
        'page title',
        'menu Title',
        'manage_options',
        'my_custom_menu',
        'my_custom_menu_page',
        '',
        101
    );
    add_submenu_page(
        'my_custom_menu',
        'page subtitle',
        'sub menu',
        'manage_options',
        'my_custom_submenu',
        'my_custom_submenu_page'
    );
    add_submenu_page(
        'my_custom_menu',
        'page subtitle2',
        'sub menu2',
        'manage_options',
        'my_custom_submenu2',
        'my_custom_submenu_page2'
    );
}

function my_custom_menu_page()
{
    echo 'Admin Page Test';
}

function my_custom_submenu_page()
{
    echo 'Admin SUB Page Test';
}

function my_custom_submenu_page2()
{
    echo 'Admin SUB Page 2';
}