This is my code.
Submenu URL for piklist admin page is /wp-admin/my-settings

add_action( 'admin_menu', __NAMESPACE__.'\\register_my_custom_menu_page', 10 );

function register_my_custom_menu_page() {

	add_menu_page( 'This is only a title of page', 'Title group', 'read', 'my-top-level-slug', __NAMESPACE__.'\\test', null, 6 );
    add_submenu_page( 'my-top-level-slug', 'This is only a title of submenu', 'Submenu', 'read', 'my-slug', __NAMESPACE__.'\\test2' );
}

function test() {
    echo '<h1>Header </h1>';
}

function test2() {
    echo '<h1>Podmenu</h1>';
}

add_filter('piklist_admin_pages', 'piklist_theme_setting_pages', 99);
function piklist_theme_setting_pages($pages)
{
    $pages[] = array(
      'page_title' => __('Piklist Settings', 'piklist')
      ,'menu_title' => __('Settings', 'piklist')
      ,'capability' => 'read' // allow users with the capability of read to VIEW this page.
      ,'sub_menu' => 'my-top-level-slug'
      ,'menu_slug' => 'my-settings'
      ,'setting' => 'custom_settings'
      ,'menu_icon' => plugins_url('piklist/parts/img/piklist-icon.png')
      ,'page_icon' => plugins_url('piklist/parts/img/piklist-page-icon-32.png')
      ,'default_tab' => 'General'
      ,'single_line' => false
      ,'save_text' => 'Save these settings'
    );

  return $pages;
}