The following steps and documentation should guide you through creating an In App Purchase in Itunes connect and applying this subscription to one of your issue.

Create an In App purchase Product ID using the instructions given below

(Go straight to section 'Managing In app Purchase' for screen shots)

This is the official Appstudio documentation and it explain you how to set up a free subscription:

2. after creating this In App Purchase you should have:

- Product ID (this should look like e.g. com.yourDNSnamel.subscription.6month)

Reference this Product ID in the App Mananger Build Form, Save the form and resubmit it to apple.

Build form:

3. When this will be done you will have to go to the list of his issues and edit the specific issue and add the product ID in here ,select Edit and paste the Product ID in IOS Universal In App Purchase ID field. (see screen shot attached)

This will associate your In App Purchase subscription to your issue.