By default, Popup Panel could be closed automatically. If you want to close the Popup Panel manually, please follow the two steps in below:

1) Double click on the Popup Panel to activate the edit mode, then select a component (like Button).

2) Drag the Link Point to link the component to the Popup Panel itself, where it is displayed as red background.

Here is a picture for your reference:

1

Interaction 12 – How to Close Popup Panel Manually in Mockplus?