There's two different ways that you can provide a free subscription to your customers. The key difference between them is how customers access your content.
We can consider the lack of a subscription record, a "free" subscription. For example, you can direct customers to register a free account. Your customer won't have a subscription, but they will have an account and will be able to view the content that you've set as free.
This method is great if you want your users to sign up and have an actual subscription, complete with access, but you don't want them to have to pay.
Create a subscription coupon with 100 as the Percent Off.
Your customer will claim the coupon during checkout
The customer will have a subscription record with the expiration date set 100 years into the future.