Hotline supports URLs via Internet Config. That means if you have Internet Config installed, you can command-click on an URL and the appropriate application will open up.
You can also configure Internet Config to recognize Hotline URLs, for example <hotline://hlserver.com/>. To configure, open Internet Config, click on Helpers, then Add, enter "hotline", then click "Choose Helper" and select the Hotline client application from your hard-drive.
If you don't have Internet Config, you can download it from any info-mac mirror, or at:
ftp://ftp.share.com/pub/internet-configuration/
Unfortunately, Netscape doesn't use Internet Config. However, it can still be configured to support Hotline URLs, and the included AppleScript will do just that. To set up your web-browser to work with Hotline, double-click the icon in this folder called "Setup Hotline with Netscape" Just double-click the script and let it do its stuff. Note that you only have to run this script once — not every time you want to use Netscape.