Including a Customized Initialize File with a wordpress header

It’s not clear where you need to add this additional code. But I would assume that it is in ‘include’-type files that you need to run in the portion of the generated page.

So, to do that, use the add_action('wp_head','your-function-name') to load your external function file:

 add_action('wp_head','include_my_include');
    function include_my_include() {
    include get_template_directory() . 'your-function-file.php';
    return;
}

See here: https://codex.wordpress.org/Function_Reference/get_template_directory .

This code would be in your theme’s (Child Theme, hopefully) function.php file.