如何在woocommerce的ajax结帐中自定义结帐字段

function custom_checkout_fields($fields){
    $fields['billing']['custom_field'] = array(
        'label'       => __('Custom Field', 'woocommerce'),
        'placeholder' => _x('Custom Field', 'placeholder', 'woocommerce'),
        'required'    => false,
        'clear'       => false,
        'type'        => 'text'
    );

    return $fields;
}
add_filter('woocommerce_checkout_fields', 'custom_checkout_fields');

<?php
    // Display the custom checkout field
    woocommerce_form_field('custom_field', $checkout->checkout_fields['billing']['custom_field'], $checkout->get_value('custom_field'));
?>

function save_custom_checkout_field($order_id){
    if (!empty($_POST['custom_field'])) {
        update_post_meta($order_id, 'Custom Field', sanitize_text_field($_POST['custom_field']));
    }
}
add_action('woocommerce_checkout_update_order_meta', 'save_custom_checkout_field');