Hi,
There are several eway payment plugins for HikaShop.
Supposing that you're using the one in HikaShop and not the eway rapid one, the payment occurs just before the order is created and if eway returns an error for the payment, the creation of the order is cancelled. So only orders with a successful payment can be created.
So the order id cannot be passed to eway as the order hasn't been created yet when the transaction is processed.
The current version of the eway payment plugin doesn't store the transaction number from eway in order either. However, you could add it by changing the line:
to:
$history->data = 'Transaction number: '. $eway->getTrxnNumber();
in the file plugins/hikashoppayment/eway/eway.php
We'll add a similar change on our end for the next version of HikaShop.
So existing orders there is no id or reference that you can use to match transactions and orders. However, you can find the payment with the time is was placed as it should be the same as the order creation date with a difference of maximum a few seconds.