You can only do this if you have an open white-label or the unlimited subdomains feature.
Open white labels allow you to provision any domain you like.
With unlimited subdomains, you select domain suffix (e.g. somesuffix.com) and you can set different domains, but they must all end in the suffix (e.g. pbx1.somesuffix.com, newyork.suffix.com, company name.suffix.com, etc).
For both options, you would use the host parameter to set the domain in the account xml.
If you don’t have the open white-label or unlimited subdomains feature, the app will always use the domain configured in provisioning options in your app in the cloud softphone web portal.