I don't see the Add certificate option in my Dashboard.
Heroku now supports multiple SSL certificates for a single app:
This feature is currently available to a subset of applications and will eventually be rolled out to all applications. You can verify whether your app is included by running this CLI command:
heroku labs -a <your-app-name> | grep allow-multiple-sni-endpoints
If your application is in the Common Runtime, you can enable this for your application by running this CLI command:
heroku labs:enable allow-multiple-sni-endpoints -a <app-name>
At this time, to have this enabled for Private Space apps, you must reach out to our Support team.
Note: If you've already added an SSL certificate prior to enabling this feature on the app, you may need to delete all the SSL certificates and re-upload them.