In the subscription question's configuration, click plans under Available Plans to add plans or click plans under Selected Plans to remove plans.

Plans are pulled from the pricing plans set up on Stripe or Braintree directly. The mode (live or test) you choose on your form must match the mode you created the pricing plan in on Stripe or the plan will not appear for selection.

NOTE: Subscription plan prices will not show in the form or check-out total price.
Instead, the subscription plan price will be referred to in the message below the check-out total.
E.g
You will be subscribed to:
TEST SUBSCRIPTION ($10.00 / month).

NOTE: Subscription plans are loaded from Stripe when you add them to the form. If you change the subscription in Stripe afterwards (eg. add a trial period) you will need to remove the subscription from the form and add it again. This will load the new subscription details and any new submissions will include the trial period.