Step 1. At your mailfloss app, go to the widget section (under "Apps & Plugins") and make sure to have your domain added to the white list.
Step 2. Copy your JS widget code, along with your public key.
Step 3. From your Leadpages dashboard, go to the left panel and click on "Conversion Tools". Select between "Pop-Ups" or "Alert Bars" depending on your content.
Step 4. Select the tool you want to add the widget to.
Step 5. Once there, click on "Settings" at the left side of your screen, and paste your code in the "Immediately before the closing </body> tag" field.