wyclef on "Auto Populate Menu"

How do I get the following to autopopulate the menu vs. require me to manually add pages to the menu in the Appearance admin area?

<?php wp_nav_menu('menu=testmenu&menu_class=child-navs&container=&container_class=&menu_id='); ?>

Hi wyclef!

Have you thought about using wp_list_pages() as a fallback?
https://developer.wordpress.org/reference/functions/wp_list_pages/

Unless you mean something different?

No, I'm pretty sure I want wp_nav_menu. Currently I'm using it and it's working fine as long as I use the manually populate it using the menu thing in the appearance admin area. I'm just looking to auto populate it so if I add a page I don't have to manually add it to the menu.