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.