To add the popup on your website, go to settings, and you have to install the following snippet of javascript on every page of your website right before the closing </body> tag.
Once you have added the snippet of javascript, you can customise the Behaviour of the popup according to your requirement.
To have the popup in your website, edit the popup and selecting the Targeting Section. Under the Targeting, paste your website URL and Submit the popup.
Keep in mind to add .* before the URL of the website, and keep the status active.
In this way, you can integrate the popup on your website and grow your subscribers base.