WooCommerce: Add Custom Checkout Field

/**
 * Add a custom field to the checkout billing section.
 */
add_action( 'woocommerce_after_checkout_billing_form', function( $checkout ) {
    woocommerce_form_field( 'company_vat', array(
        'type'        => 'text',
        'class'       => array( 'form-row-wide' ),
        'label'       => __( 'VAT Number', 'theme-domain' ),
        'placeholder' => __( 'e.g. GB123456789', 'theme-domain' ),
        'required'    => false,
    ), $checkout->get_value( 'company_vat' ) );
} );

/**
 * Validate the custom field.
 */
add_action( 'woocommerce_checkout_process', function() {
    $vat = isset( $_POST['company_vat'] ) ? sanitize_text_field( wp_unslash( $_POST['company_vat'] ) ) : '';
    if ( $vat && ! preg_match( '/^[A-Z]{2}[0-9A-Z]{2,12}$/', strtoupper( $vat ) ) ) {
        wc_add_notice( __( 'Please enter a valid VAT number.', 'theme-domain' ), 'error' );
    }
} );

/**
 * Save the custom field to order meta.
 */
add_action( 'woocommerce_checkout_update_order_meta', function( $order_id ) {
    if ( ! empty( $_POST['company_vat'] ) ) {
        update_post_meta(
            $order_id,
            '_company_vat',
            sanitize_text_field( wp_unslash( $_POST['company_vat'] ) )
        );
    }
} );