$menu_slug = "my_menu_slug";
$desired_capability = "manage_options"; //Or whatever you need
$menu_page_callback = "menu_page_callback_function";

add_menu_page(
    "Page Title",
    "Menu Title",
    $desired_capability,
    $menu_slug,
    $menu_page_callback
);

add_submenu_page(
    $menu_slug,
    "Submenu Page Title",
    "Submenu Menu Title",
    $desired_capability,
    $menu_slug,
    $menu_page_callback
);
//Note, the 5th and 6th parameters here are the same as above. This
//overrides the first submenu title. Now clicking the
//menu title and the first submenu item will navigate to the same page,
//generated by the menu_page_callback function.