This dialog lets you do that. Each word in the text widget is an applet group name. If the group name does not begin with a period, it is a host name, and the group consists of exactly that host. An exact host name match is preferred over a domain name match. If no host name matches, the longest matching domain name is chosen.
All comparisons are case insensitive.
If a server's name matches none of the given host or domain names, it becomes a group by itself. A host or domain name should not end in a period. Without a secure transport protocol, a known site or domain can never be completely trusted, except for other hosts within the same firewall. Using DNS spoofing or other techniques, a rogue site anywhere on the Internet could masquerade as a known site outside the firewall. Never trust an applet with sensitive data, except when you live inside a firewall and the applet lives within that same firewall.