Good afternoon Robert,
In your link instead of creating a basic link like 'index.html', you can add the discount code into the link by concatenating it onto the end of the link value via an ampersand.
If the voucher code never changes, you can hardcode this into the link. Let's say the discount code is Xp91dMe, you could do:
<a href='index.html&code=Xp91dMe'>Click here for a discount!</a>
Then, whatever page that directs to, you will need to use a serverside language to process the link (i.e - make sure the discount code in the URL matches the actual discount code), then you can decide what to do from there depending on the value.
Are you doing something similar to affiliate marketing?
Hopefully someone else can add to my advice!