Create the desired elements:

• A button that triggers the appearance of the pop-in,

• The pop-in itself (it can contain different content - images, videos, text etc.),

• A button to “Close” the popin.

For the buttons you can use the Rectangle Frame Tool (F). If the buttons contain multiple elements (like a rectangle frame for the background, a label, an icon, etc.) you should group all these elements (select them, then right click and select Group)

Once the selection is grouped you can convert it to a button. Go to Window -> Interactive -> Buttons and Forms and with the selected group set the Type to Button.

Repeat the same process for the "Close" button.

Now it's time to group the "Close" button and the content of the pop-in. Right click the selection and choose Group.

With the content and the close button group selected go to Window -> interactive -> Object states and create a new multi-state object. (In this example it's important to select a single group and not multiple objects, because otherwise each object in the selection will become a separate state and we don't want this.)

Two states are now created. You can rename the last state to "Closed".

Draw an empty rectangle somewhere on the page. This rectangle will help us select the multi-state object even when the active state is "Closed".

Open Window -> Layers (F7), drag and drop the empty rectangle inside the Closed state of the Multi-state object.

Delete the other contents of this state, so that only the empty rectangle shows-up in the closed state.

Select State 1 of the multi-state object. Double click on the "Close" button to enter the Group and select it. You can also achieve this from the Layers window by clicking on the little square to the right side of the "Close" layer.

Go to Buttons and forms and set an action for this button to Go To State and choose Closed.

Now select the button which will open the Pop-in and add an action for it with type Go To State and choose State 1 from the states.

You may wish to move (drag-and-drop) the "Closed" state at first position if you wish for the pop-in to be initially closed.

Go to Window -> Interactive -> EPUB Interactivity Preview and click the Play icon to preview the result. If you are happy with the result, save the page and upload it to our server.

If you wish to have multiple Pop-ins on the page you can create a new state for each of the pop-ins you desire. You can even add an animation for each of the states.

Here is the final result.