The Domain/Host field in Zoiper always needs the IP or the domain name of the VoIP provider's server. Usually the provider will provide you with details for your VoIP account and Proxy/Host/Domain, something looking like:
your username - my_username
your password - my_password
proxy / host / domain / registrar - sip.something.com or something.com- this goes to Zoiper's Host/Domain field.
Sometimes you might also see "Outbound proxy" provided along with the details above, which goes to Zoiper's "Outbound proxy".