At my last host I managed to do this but can’t quite remember how I did it.
I had my service domain on a windows server and a subdomain of it on a unix server. I don’t think I used a dedicated ip. Any ideas on how I can achieve this?
you can do subdomain of your service domain, it is called a third level domain 
I tried that but it told me that zone was taken. Any idea why?
Hrm, you would need to submit a ticket if that is the case.