To add a submenu under a custom menu in the WordPress admin panel, you can use the built-in add_submenu_page() function. Here’s how:

1. Open functions.php from your theme directory, or build a plugin.
2. Paste the following code into the file:

// Top-level admin menu
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');

// Submenu below custom 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 page callback
function custom_menu_callback() {
    echo '<div class="wrap">';
    echo '<h1>Custom Menu</h1>';
    echo '<p>Welcome to your custom admin menu page.</p>';
    echo '</div>';
}

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

Once added, you’ll see the “Custom Menu” with its “Submenu” below it inside the admin dashboard. Customize content as needed.