Click on the Payments tab > then the Products
Choose the Product that you wanted the membership to be added to
Inside the product settings, Go to the Pricing tab then click on the Product
On the products pricing settings, scroll down to the Pricing section then click on the Additional Options to expand
Toggle on the Membership Offer then select the offer you wanted to include in the product
Scroll down and click Save
Once this is setup and the payment is made for the said product, there is no need for any additional setup. The user will be automatically added to the course.