If I use the softaculous server site to add an application to a site I am able to specify a 'Server Host (Optional)' - this allows me to have a domain.tld that resolves to a different IP to ftp.domain.tld.
The API does not appear to support this. If I specify my 'domain.tld' I get 'Could not Resolve Domain Name', but if I specify 'ftp.domain.tld' I get 'The FTP path submitted is incorrect and does not point to the given domain.'