Add custom hidden checkout field with a default value in woocommerce

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');

Hidden type doesn't exist buy default for woocommerce form fields… But you can create it.

• The first function will create that hidden field type.
• The 2nd function will add this hidden custom field (where you will set the value)
• The third function will display this value in order edit page

Here is the code:

// Create hidden checkout field type
add_filter( 'woocommerce_form_field_hidden', 'create_checkout_hidden_field_type', 5, 4 );
function create_checkout_hidden_field_type( $field, $key, $args, $value ){
    return '<input type="hidden" name="'.esc_attr($key).'" id="'.esc_attr($args['id']).'" value="'.esc_attr($args['default']).'" />';
}

// Add custom hidden billing checkout field
add_filter( 'woocommerce_checkout_fields', 'custom_billing_fields' );
function custom_billing_fields( $fields ){

    ## HERE set the value (for this hidden checkout field)
    $value = "The value";

    $fields['billing']['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,
        'default'   => $value, // The custom field value
    );
    return $fields;
}

// Display the field value on the admin order edit page (after billing address)
add_action( 'woocommerce_admin_order_data_after_billing_address', 'display_custom_field_in_admin_order_meta', 10, 1 );
function display_custom_field_in_admin_order_meta($order){
    echo '<p><strong>'.__('Quickbook').':</strong> ' . get_post_meta( $order->get_id(), '_billing_quickbook', true ) . '</p>';
}

The Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

USAGE (RETRIEVING THE VALUE):

To get the field value from the WC_Order Object $order, you will use (compatible with HPOS):

$value = $order->get_meta('_billing_quickbook');

This code is tested and works in WooCommerce 3+. Doesn't Work with new checkout Blocks.

Official developer documentation: Customizing checkout fields using actions and filters