In order to call a plugin from your business object you need to add the plugin to your business object first (see Create Server Side Business Objects) and afterwards access it within a Business Object Function using this syntax:

Simplifier.Plugin.<PluginName>.<SlotName>(payload?: string|object): object

Example:

var repos = Simplifier.Plugin.contentRepoPlugin.listRepos(); 
var newRepo = Simplifier.Plugin.contentRepoPlugin.createRepo({name: "MyRepo"}); 
Simplifier.Plugin.contentRepoPlugin.updateRepo("{\"id\": 15}");