Customize register form to add fields

woocommerce_register_form

function tzn_woocommerce_billing_fields($billing_fields) {
    $fields = "";
    if ($fields && is_countable($fields)) {
        foreach ($fields as $key => $field) {
            if (is_user_logged_in() && $key == 'billing_email_2') {
                continue;
            }

            $billing_fields[$key] = $field;
        }
    }

    return $billing_fields;
}

add_filter('woocommerce_billing_fields', 'tzn_woocommerce_billing_fields', 9999, 2);
/* Add */
function tzn_add_extended_fields() {
    wp_enqueue_script('wc-country-select');
    wp_enqueue_script('wc-address-i18n');
    wp_enqueue_script('wc-checkout');
    if (!isset($_GET['handler']))
        echo '<p class="title_billing_title">' . __("Persönliche Informationen", "tzn") . '</p>';
    $checkout = WC_Checkout::instance();
    $fields = $checkout->get_checkout_fields('billing');
    $skip_fields = array('billing_email', 'billing_email_2');
    if (isset($_GET['handler']))
        $skip_fields = array('billing_title', 'billing_email', 'billing_email_2');
    foreach ($fields as $key => $field) {
        if (in_array($key, $skip_fields)) {
            continue;
        }

        if (isset($field['country_field'], $fields[$field['country_field']])) {
            $field['country'] = $checkout->get_value($field['country_field']);
        }
        woocommerce_form_field($key, $field, $checkout->get_value($key));
    }
}

add_action('woocommerce_register_form', 'tzn_add_extended_fields', 1);

/* Save */
function tzn_save_extended_fields($customer_id) {
    if (!isset($_GET['handler'])) return;
    if (defined('REST_REQUEST')) {
        return true;
    }
    $checkout = WC_Checkout::instance();
    $fields = $checkout->get_checkout_fields('billing');
    foreach ($fields as $key => $field) {
        if ($key == 'billing_email') {
            continue;
        }

        if (!isset($_POST[$key])) {
            continue;
        }

        update_user_meta($customer_id, $key, sanitize_text_field($_POST[$key]));
        if ($key == 'billing_first_name') {
            update_user_meta($customer_id, 'first_name', sanitize_text_field($_POST[$key]));
        }
        if ($key == 'billing_last_name') {
            update_user_meta($customer_id, 'last_name', sanitize_text_field($_POST[$key]));
        }
        if ($key == 'unternehmen') {
            update_field( 'unternehmen', sanitize_text_field($_POST[$key]),'user_' . $customer_id);
            update_user_meta($customer_id, 'unternehmen', sanitize_text_field($_POST[$key]));
        }
        if ($key == 'ust_idnr') {
            update_user_meta($customer_id, 'ust_idnr', sanitize_text_field($_POST[$key]));
            update_field( 'ust_idnr', sanitize_text_field($_POST[$key]),'user_' . $customer_id);

        }
        
    }
    if (isset($_POST['billing_title'])) {
        update_user_meta($customer_id, 'billing_title', wc_clean($_POST['billing_title']));
    }
}

add_action('woocommerce_created_customer', 'tzn_save_extended_fields');
add_action('woocommerce_save_account_details', 'tzn_save_extended_fields');