Yes you do need certificates for HTTPs.
You’ll have to adjust your vhost - you can find examples for that in /opt/zammad/contrib/ and then generate an certificate.
I’d like to suggest to this with a subdomain.
All you need to know is written down in the documentation:
https://docs.zammad.org/en/latest/getting-started/configure-webserver.html
Make sure to also adjust FQDN and HTTP-Type in the Zammad settings.