Swish - Change paymentreference

functions.php

<?php
/**
 * @snippet       WooCommerce - Change Reference
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 3.9.0
 */
function redlight_swish_custom_reference( $data, $order ) {
  $ref =  $data['payeePaymentReference'];
  $data['payeePaymentReference'] = 'swish-'.$ref;

  return $data;
}
add_filter('redlight_swish_ecommerce_paymentrequest_data', 'redlight_swish_custom_reference', 10, 2 );