User authentication is not done through the SDK, but rather through Zoom OAuth. For more information on Zoom OAuth, please see our documentation. If you have any questions related to implementing OAuth, you can use the #api-and-webhooks:authentication category.
To get a user’s ZAK, you will need to use the REST API with the access token you obtained from OAuth. If you run into any issues with this, you can use the #api-and-webhooks category.