This [FQDN] domain doesn't resolve to an IP.
So all IP based authentications are not possible for it (like HTTP & HTTPS).
Which leaves only DNS authentication for such a name.
If you haven't done DNS authentication before on DSM, then you need to get that "how to" information from the web or Synology.
[I don't think you will find such a device specific setting (or "how to") in this forum - but you are free to look]
[EDIT - if you do find "how to" do that, maybe you could post a link to it here ]