I have updated the the "ZertoAppliance" modules to support the Linux appliance authentication, with help from Bhavesh. The updated files are in the Github repo.
zertoappliance.api.id and zertoappliance.api.key set the client id/client secret credentials configured in Keycloak: https://help.zerto.com/bundle/Linux.ZVM.HTML.10.0/page/Creating_Keycloak_Credentials.htm
zertoappliance.port is set to 443 by default, and this automatically chooses the Linux authentication method. Setting any other port will choose the Windows authentication method, 9669 is the default for Windows.
If there is a Windows appliance running on port 443 (or Linux not running on 443) zertoappliance.type can be used for force the authentication method to either "linux" or "windows".