在woocommerce中添加带有默认值的自定义隐藏结帐字段

Question

How to add custom field in checkout Woocommerce with hidden type and default value?

please check below my code:

function pord_checkout_fields( $fields ) {
    $fields['billing']['quickbook'] = array(
        'type' => 'hidden',
        'label'     => __('Purchase Order Number', 'woocommerce'),
        'placeholder'  => _x('Purchase Order Number', 'placeholder', 'woocommerce'),
        'required'  => false,
        'class'     => array('form-row-wide'),
        'clear'     => true
     );

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