Code

add_action('admin_menu', 'register_my_custom_submenu_page');
 
function register_my_custom_submenu_page() {
    add_submenu_page(
        'tools.php',
        'Submenu Page',
        'My Custom Submenu Page',
        'manage_options',
        'my-custom-submenu-page',
        'my_custom_submenu_page_content' );
}
 
function my_custom_submenu_page_content() {
    echo '<div class="wrap">';
        echo '<h2>Page Title</h2>';
    echo '</div>';
}

Output

Explanation

In the code, we created a function named register_my_custom_submenu_page and we used add_submenu_page to add the item to the navbar as a child of tools.php, which is the Tools page.

Please check the parameters part in this page to know about the arguments we passed in. Then we used add_action to run our register_my_custom_submenu_page function. Finally, we created the function my_custom_submenu_page_content to display contents in the page.