A customer add the voucher to the product in the checkout, in the first purchase step.
1. Add the voucher:
Go to the product. Press buy.
Press the "I have a voucher button".
Enter the voucher and press "apply voucher".
2. Preview of voucher
When the voucher has been added correctly you will get a preview of the discount.
You will get an error saying "the voucher is not valid" if you try to enter a expired voucher or if you misspelled the voucher.
3. Continue the purchase steps
Create an account or log in.
Follow the steps.
For a voucher on a subscription the customer will always need to fill in their card information.