To Add a Drawer to a Panel

Desktop User Guide
Using the Panels
Drawers

You can add a drawer to a panel in the following ways:

From the panel popup menu

Right-click on any vacant space on the panel, then choose Add to Panel.
In the Add to Panel dialog, select Drawer. Click Add, then click Close.

From any menu

You can add a menu as a drawer object to a panel.

To add a menu as a drawer to a panel, open the menu from the panel. Right-click on any launcher in the menu, then choose Entire menu ▸ Add this as drawer to panel.