Add code to your child theme’s
functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s
functions.php file as this will be wiped entirely when you update the theme.
Note that the
default_checkout_billing_country filter affects both existing and non-existing users. If you want to only change the default for non-existing users, then you can use this: