using-add_submenu_page.php

function custom_submenu_page() {
    add_submenu_page(
        'options-general.php', // Parent menu slug
        'Custom Submenu Page', // Page title
        'Custom Submenu', // Menu title
        'manage_options', // Capability
        'custom-submenu-page', // Menu slug
        'custom_submenu_page_callback' // Callback function
    );
}
add_action( 'admin_menu', 'custom_submenu_page' );

function custom_submenu_page_callback() {
    // The HTML content and logic of your submenu page goes here
}