To include a custom submenu item in the WordPress admin dashboard, you can use the add_submenu_page() function. Follow these instructions:

1. Open your theme’s functions.php file or create a plugin file.
2. Insert the following code snippet into the file:

// Add a top-level menu item
function custom_menu_page() {
    add_menu_page(
        'Custom Menu', 
        'Custom Menu', 
        'manage_options', 
        'custom-menu', 
        'custom_menu_callback', 
        'dashicons-admin-generic', 
        25
    );
}
add_action('admin_menu', 'custom_menu_page');

// Add a submenu under the top-level menu
function custom_submenu_page() {
    add_submenu_page(
        'custom-menu', 
        'Submenu', 
        'Submenu', 
        'manage_options', 
        'custom-submenu', 
        'custom_submenu_callback'
    );
}
add_action('admin_menu', 'custom_submenu_page');

// Main menu page content
function custom_menu_callback() {
    echo '<div class="wrap">';
    echo '<h1>Custom Menu</h1>';
    echo '<p>This is your custom menu page.</p>';
    echo '</div>';
}

// Submenu page content
function custom_submenu_callback() {
    echo '<div class="wrap">';
    echo '<h1>Submenu</h1>';
    echo '<p>This is your submenu page.</p>';
    echo '</div>';
}

Save your changes and reload your admin panel. A new menu titled “Custom Menu” will appear, with a submenu named “Submenu” beneath it.