In order to do what you want, you can use the custom field by using an input box like
<input type="text" name="custom" value="woocommerce-code"></input> and the text in the value field is what will be replaced with the code used.

Since I was not a code guy, I would ask a silly question further. After I got your answer I did a search with those results and the same thing is everywhere but no one says, in which file these code should be written. I found there is lot files in woo commerce plugin and checked couple of them like (webhook.php, rest.php, and orders.php) but no luck. Is there way you can help me with this?