On submit add something to a form

Using javascript (or jQuery) how can I add the word "Success" to "&resultCode=" on form submit? So the link should look like this after submit:

ht tps://somesite.com/payment.php?Done&sellerOrderId=<?php echo $item_number; ?>&resultCode=Success

<form action="https://www.paypal.com/cgi-bin/webscr" method="post" id="myForm">

<input type="hidden" name="return" value="https://somesite.com/payment.php?Done&sellerOrderId=<?php echo $item_number; ?>&resultCode=">

<button...

Code (markup):

On submit add something to a form

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