Follow these instructions to add your item to a popup placement and then add it to your site.
To add the item to a popup placement
Go to the Placement dashboard
Enable the Popup placement
Click the Select Item button to select the item you want to add to the popup
Click the Settings button to define the attributes of the popup (e.g. exit popup)
Click the Embed button and copy the script code
To embed the popup to a post/page
If you are using the Gutenberg post/page editor, add a Custom Html block to your post/page and paste the script code you copied
If you are using the Classic post/page editor, paste the script code you copied to the Text tab of the editor
To add a popup to the whole WP site
In case you want to add your popup placement to all the pages/posts on your WordPress site, you can use the following plugins:
1) Free option - Insert header and footer plugin - note that using this plugin you'll be able to integrate the popup placement code in all the posts/pages only
2) Paid option - Header and footer script loader plugin - with this plugin you can also customize which pages display the placement code / widget and which don't