Is it possible to register CA with AWS IoT Core without having access to Private Key ?

Let's say we are getting our device certificates signed by 3rd party CA e.g digicert. Can we register this 3rd party CA on AWS IoT Core for JITP without having access to Private Key ? As the registration process requires the proof of ownership. [https://aws.amazon.com/about-aws/whats-new/2022/07/aws-iot-core-provision-iot-devices-aws-accounts-registrations-certificate-authorities/](This Blog) seems to suggest that it's possible but did not find details anywhere on the workflow.