The Activate Subscriptions call requires a content body.

This action will set the status of multiple Subscriptions to Active, which in most cases this will generate an invoice, and depending on the customer's billing options, apply a charge and make a collection attempt against the Customer's payment method. The activations occur regardless of the collection attempt succeeding or failing, unless the field invoicePreview is set to true.

All subscriptions being activated from this request are required to belong to the same customer. This action can only be performed when all Subscriptions have a status value of Draft or Provisioning.

This call returns a list of the Subscriptions which you have activated so you can verify that the status field is set to Active. If previewing, then the response also includes a preview of the invoice that would be generated.