After you get done setting up a voucher, you can create and send customers a URL with a special message to it so that the voucher will automatically get added to their cart (and if your voucher is setup to automatically add a plan or bundle to cart, that will show up, too!).
Creating the link is easy and would be formatted l like this:
https://mygreatwebsite.com/?voucher=voucher123
Just make sure you replace voucher123 with the name of the voucher code you created.
Additionally, if you want to automatically open the cart so it not only automatically adds the voucher (and possibly plan/bundle), you can add one more bit of text to the URL to also start the checkout:
&show_cart=true
So the full URL would look like:
https://mygreatwebsite.com/?voucher=voucher123&show_cart=true