Add Merchant Data

You can append metadata to existing transaction's mercant_data container (overwrites existing value atm).
Returns updated transaction object.
<?php

$transaction_id = '0368156c-...-282f7eada86a';
$request_body = array('merchant_data' => 'my new metadata');

$transaction = $MK->addTransactionMeta($transaction_id, $request_body);

?>

Required fields are underlined.