@Hakan Thank you for your question.
You are right that the D1 tier supports custom domains. Unfortunately though, the D1 tier does not support SSL bindings for the custom domain. In order to use an SSL certificate to secure your custom domain, you will need to use the basic tier or higher.
For more information on what features are available with each tier, please refer to this chart for more information.
Let us know if there are any further questions or concerns.