Adding submenu to custom plugin menu page created with add_menu_page() function

by

You may take use of add_submenu_page function:

Add a top-level menu page

add_menu_page(
    $page_title,
    $menu_title,
    $capability,
    $menu_slug,
    $function,
    $icon_url,
    $position
);

Add a sub-menu page

add_submenu_page(
    $parent_slug,
    $page_title,
    $menu_title,
    $capability,
    $menu_slug,
    $function
);