How do I create a Website Lightbox Sign-Up Form?

In your Contacts page, create a new list for this form specifically
Click the Sign-Up Form icon located in the left navigation panel and then click Add Sign-Up Form
In the Contact List drop-down, select your list
Click Save and go forward to design the sign-up form
After you are done with the design, go through the rest of the steps. In Settings, set up the parameters for your sign-up form.
Paste the code into the <body> portion of your website