If you want to access a namebased website that you created e.g. in ISPConfig before the DNS records are setup, edit the hosts file on your workstation computer and add a line like this:
Replace the IP address with the IP of your server and www.mydomain.com with the domain name of the website.
The hosts file on linux and MAC systems is /etc/hosts, on Windows systems the hosts file is C:\Windows\system32\drivers\etc\hosts
The above trick also works if you want to run a website in your internal network (intranet) without having a DNS server availalble for local dns resolving.