How to apply 2D effects is described here: https://doc-snapshots.qt.io/qtdesignstudio-4.14/quick-components.html.

 
This is the current text:
To apply a visual effect to a component, you must place the component inside the effect. First drag-and-drop an effect from Library > Effects to the the Form Editor or the Navigator, and then drag-and-drop the component to the effect. For some effects, you need two source components.
 
Since Qt Design Studio 2.0 the effects are a child of the item they affect. To add an effect you have to drag the effect from the item library on top of the target item in the navigator.