create drop down menu

Question

I need to add a dropdown menu like the entries in Wordpress, I am adding this code I have managed to add the parent item to the left side menu, but I need the user to do a Mousehover appear more menu options, I am learning to create plugins and I have this Doubt, I appreciate your support.

Example:

function my_plugins(){
    add_menu_page( 
        'My Plugin',
        'My Plugin',
        'manage_options',
        'visitor-counter-by-funcion',
        'my_custom_menu_page',
        'dashicons-chart-bar'
    ); 
}
add_action( 'admin_menu', 'my_plugins' );

/**
 * Display a custom menu page
 */
function my_custom_menu_page(){
    esc_html_e( 'Admin Page Test', 'textdomain' );  
}

Answer 1

What you are searching for its a submenu page. Try with this:

add_submenu_page( 'my-top-level-slug', 
                  'My Custom Submenu Page', 
                  'My Custom Submenu Page',
                  'manage_options', 
                  'my-secondary-slug'
);

you can find more info here

An example with your code:

function my_plugins() {
    add_menu_page(
            'My Plugin', 
            'My Plugin', 
            'manage_options', 
            'visitor-counter-by-funcion', //THIS IS THE SLUG YOU NEED TO USE IN YOUR SUBMENU ITEMS
            'my_custom_menu_page', 
            'dashicons-chart-bar'
    );
    add_submenu_page(
            'visitor-counter-by-funcion', //THIS IS THE SLUG OF YOUR PARENT MENU PAGE
            'My Custom Submenu Page', 
            'My Custom Submenu Page', 
            'manage_options', 
            'my-secondary-slug', 
            'my_custom_sub_menu_page'
    );
}

add_action('admin_menu', 'my_plugins');

/**
 * Display a custom menu page
 */
function my_custom_menu_page() {
    esc_html_e('Admin Page Test', 'textdomain');
}

/**
 * Display callback for the submenu page.
 */
function my_custom_sub_menu_page() {
    ?>
    <div class="wrap">
        <h1><?php _e('My Custom Submenu Page', 'textdomain'); ?></h1>
        <p><?php _e('My Custom Submenu Page', 'textdomain'); ?></p>
    </div>
    <?php
}

its looking like this: