add_filter( 'woocommerce_bacs_account_fields', 'custom_bacs_account_field', 10, 2);
function custom_bacs_account_field( $account_fields, $order_id ) {
$order = wc_get_order( $order_id );
$account_fields['variable_symbol' ] = array(
'label' => 'Variabilní symbol',
'value' => $order->get_order_number()
);
return $account_fields;
}