1. Go to Short Link.
2. Beside the short link you wish to add an interstitial popup, click on the "..." button.
3. Select "Edit Link".
4. Select "Interstitial Popup" on the navigation menu on the left.
5, At the Interstitial Popup section, enter in the desired customization for your popup.
6. Finally, click on "Add First Popup" to publish the changes.
Please note that some website may not permit an interstitial popup. For websites that permit it, you should see the page loading in the preview.