Redirection to https

I have a lock at this address
Authorization of a Microsoft 365 account requires a return address with https
Can this be changed quickly? The installation is local, I do not need certificates

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:

Make sure to also adjust FQDN and HTTP-Type in the Zammad settings.