Your extension can take configuration parameters as passed via the "Service management" control panel. The following screenshot shows a Bugzilla extension being configured with a "bugzilla_url" parameter:

Your extension can then reference the configuration parameter by accessing config["configuration_parameter"]. For example, to access and create a link using the bugzilla_url parameter, your DekiScript would look like:
<a eval:href="config['bugzilla_url'] .. '/show_bug.cgi'">show bug link</a>