A: The opportunity needs a product under the product type "Subscription":

Subscriptions require the Subscriptions add-on.

A popup gives you the option to create a subscription when the opportunity is moved to a stage with 100% probability. Turn on Create subscription from this order and click Close deal. If you close the opportunity with the Won button, click Continue to create a subscription instead.

The new order will then be counted as the first order that this subscription creates.

If the company already has active subscriptions, the popup instead lists them under Update active subscription with. Choose a subscription to add the products to it, or click Create new subscription or Create independent order.

You can also create the subscription later: open the order and click Create subscription.