Two-part form open into Lightbox?

I have successfully followed your tutorial to create a two-part contact form in which some data from Form 1 is carried into Form 2. However instead of clicking Form 1 “submit” resulting in Form 2 appearing on a new actual page, I want a lightbox to open containing Form 2. How can I code this? I am using Form Lightbox plugin. URL with test forms: http://www.tellasia.org/newsite/wordpress/about-us and http://www.tellasia.org/newsite/wordpress/contact
thanks for your help!

https://wordpress.org/plugins/si-contact-form/