This command can be run from a Powershell terminal on the Windows DNS server:
dnscmd [<servername>] /zoneexport <zonename> <zoneexportfile>
The arguments for the command are:
servernameSpecifies the DNS server to manage, represented by IP address, FQDN, or host name. If this parameter is omitted, the local server is used.
zonenameSpecifies the name of the zone.
zoneexportfileSpecifies the name of the file to create.
There is also the offical documentation from Microsoft.
This command will generate a file that is named what you provided for
zoneexportfile and it will be located in the
%systemroot%/System32/Dns directory by defaullt.
Once you have a zonefile generated, you can continue with your discovery by following the DNS Discovery Guide.