I did not register mine. I just used the IP Address and not FQDN. After the upgrade is complete you won't need the number again. Here's some documentation: https://docs.vmware.com/en/VMware-vSphere/7.0/com.vmware.vcenter.upgrade.doc/GUID-24D34C53-B00E-47B7...
My upgrade (in only one particular environment) has failed x3 because the hostname cannot be looked up via PTR record:
2023-01-13T18:29:38.681Z appliance.net.addr is set to '172.16.1.121'
2023-01-13T18:29:38.681Z appliance.net.prefix is set to '24'
2023-01-13T18:29:38.681Z appliance.net.gateway is set to '172.16.1.150'
2023-01-13T18:29:38.681Z Prefix is not 32, no need to update /etc/systemd/network/10-eth0.network
2023-01-13T18:29:38.681Z Getting value for install-parameter: appliance.net.dns.servers
2023-01-13T18:29:38.681Z DNS servers: 172.16.1.1, 172.16.1.239
2023-01-13T18:29:39.464Z Getting value for install-parameter: appliance.net.dns.searchlist
2023-01-13T18:29:39.464Z DNS search:
2023-01-13T18:29:39.464Z Getting value for install-parameter: appliance.net.mode
2023-01-13T18:29:39.464Z Found static mode. No need to wait for IP.
2023-01-13T18:29:39.464Z Getting value for install-parameter: appliance.net.pnid
2023-01-13T18:29:39.464Z Executing command: ['/opt/vmware/share/vami/vami_set_hostname'] with retryCount: 5, waitTimeInSecs: 15
2023-01-13T18:29:39.464Z Running command: ['/opt/vmware/share/vami/vami_set_hostname']
2023-01-13T18:29:40.628Z Done running command
2023-01-13T18:29:40.628Z Stdout: DNS reverse lookup on 172.16.1.121 has failed.
Unable to obtain hostname from DNS reverse lookup.
Please exam DNS/network configuration.
Skip setting hostname.