How can I add a lightbox for a newsletter?

I'd like people to be able to click a button that opens a lightbox to sign up for my newsletter. I'm currently using the form block instead which allows a lightbox, but doesn't allow integration with Squarespace email campaigns, and I don't want to use mailchimp.

Does anyone know of the CSS? (Or another solution?)

Thank you!

I second this question! 

Would also love to know if anyone has a workaround for the integrating the form block into email campaigns/subscriber lists?

You can use this anything lightbox plugin to create newsletter popup.

The idea here is to insert the Newsletter Popup into the page (any position), then use a plugin to create a lightbox for it.

You can also use this free tool to build custom popup.