To Add an Opt-In button to WordPress it's very easy. Follow the above steps:
1) Edit your page and click AddWidget button
2) Choose Custom HTML Widget
3) Edit your campaign and go to Installation Step
4) Change the following settings
Position to LightBox
Show Exit Intent to OFF
Show On Specific Website Pages to OFF
Display Frequency to Everytime
5) Copy the code from the second text area and paste it into Custom Html Content Fields.
6) Give Custom Html Widget a name and click on Done button.
7) Test your page, you should see the following result