In the agent configuration file, when leaving Hostname undefined (it's commented out), reads HostnameItem to determine the system's hostname. I took advantage of this situation in the following way:

Configuration file snippet:

# Hostname=Windows host
HostnameItem=system.run[powershell -NonInteractive "[System.Net.Dns]::GetHostByName(($env:computerName)).Hostname.ToLower()"]

This works OK for zabbix_agentd.exe, but does not work for zabbix_sender.exe:

Program output:

C:\Windows\system32>"%ProgramFiles%\zabbix_sender" -v -c %ProgramData%\Zabbix\Agent\zabbix_agentd.conf -k test -o "%Date% %Time%"
zabbix_sender [4920]: Warning: 'Hostname' parameter required
sent: 0; skipped: 1; total: 1

Expected behaviour: zabbix_sender.exe falls back to HostnameItem if Hostname is undefined like zabbix_agentd.exe does.