add_dashboard_page

add sub menu on dashboard menu, a page given.

function my_plugin_menu() {
    add_dashboard_page('My Custom Dashboard', 'Custom Dashboard', 'edit_posts', 'my-custom-dashboard', 'my_custom_dashboard_output');
    add_dashboard_page('My Custom Dashboard', 'Halo Dashboard', 'edit_posts', 'my-custom-', 'my_custom_dashboard_output');
}
add_action('admin_menu', 'my_plugin_menu');

function my_custom_dashboard_output() {
    echo 'Welcome to My Custom Dashboard!';
}