it's not inside the code.And where exactly is one to insert the random purchase code? here <h5><?php _e('Purchase Code') ?></h5> or 'purchase_key'
It is located in url.
For example go to this url :
yourdomain[DOT]com/admin/global/settings.php#quick_purchase_code
Replace "yourdomain[DOT]com" with yours.
Dont forget to login as admin first