You will need to create a SetupIntent and collect authentication at the time the customer signs up so the stored card information can be reused with the Merchant Initiated Transaction (MIT) exemption applied in the future.
If this is not done, then the customer will be prompted to authenticate at the time of charge and you will then need to go through a recovery flow.
Subscriptions created using Stripe Billing will have a SetupIntent automatically created in preparation for future charges. This SetupIntent will ask your customer to authenticate while on session so that when the trial comes to an end, the customer does not need to be brought on session again to confirm their details.
The first charge that creates a subscription must be authenticated using 3D Secure, even if it is a free trial. Banks do not require this prior to Strong Customer Authentication (SCA)'s effective date of September 14th, but this will be required after SCA has gone into effect.
Subscriptions and stored card information prior to the SCA rollout on September 14th will have the MIT exemption automatically applied.