xfce4-panel - Add New Items
The Add New Items window allows you easily add new plugins onto your panel. It can be accessed by right-clicking on the desired panel and selecting Panel → Add New Items….
|Add New Items window|
Adding Panel Plugins
To add a plugin to a panel, select a plugin from the list and then click Add. The plugin you have added will appear at the end of the selected panel. You can use the search box to search for your desired plugin in the list.