Sure, what are you struggling with? The default selected property is a boolean that can be set on your entity. If this is set to true, the object will be selected by default or when it is reset. Sometimes this property is hard to use, e.g. if you cannot store this isDefault boolean in the database (e.g. when it is user specific). In that case you can also select a string where the titles of the default selected objects are stored in json format. Also note that there is a demo project on github in the test folder.
One other remark. If you don’t need all advanced options, like loading the content when opening the widget, there is also a new widget from mendix now that might be interesting: Mendix Marketplace - Searchable Reference Selector
Hope this helps!