Insert link in sub menu – Admin panel

You can try this.

add_action( 'admin_menu', 'register_my_custom_menu_page' );
function register_my_custom_menu_page() {
    add_menu_page(
        'My Custom Page',
        'My Custom Page',
        'manage_options',
        'my-top-level-slug',
        '',
        'dashicons-menu',
        6
    );
    add_submenu_page( 'my-top-level-slug', 'My Custom Page', 'My Custom Page',
    'manage_options', 'my-top-level-slug', 'custom_subpage_first');
    add_submenu_page( 'my-top-level-slug', 'My Custom Submenu Page 1', 'My Custom 
    Submenu Page','manage_options', 'my-secondary-slug', 'custom_subpage_second');
}

function custom_subpage_first() {
?>
    <div class="wrap">
       <?php include_once(dirname( __FILE__ ) . "/file.htm");?>
    </div>
<?php
}
function custom_subpage_second() {
?>
    <div class="wrap">
       /*Second subpage content */
    </div>
<?php
}