If you are deploying in server mode, then you must first acquire and setup a domain name through which the DMS services can be externally accessible.
The purpose of server mode is to make the DMS system easily available to external users via a custom domain name and secured over HTTPS.
Your domain name must be provided during installation of the SSL certificate and as input into the DMS interactive configuration questionnaire.
If you are part of an institution or organization, please contact your IT department for help setting this up. Alternatively, if you are an individual user without organizational infrastructure support, you can acquire your own domain name via the many legitimate providers available over the internet.