In order to create an HTML lightbox, please follow the steps below:
Go to Dashboard >> Pages >> Edit >> Create two nested columns
Set the class of the outer column to hidden. Click on Save Element button.
Set the ID of the inner column to my-lightbox-content. Click on Save Element button. You have to fill in the content that you want to show in the lightbox, in this column.
Create a lightbox shortcode.
In the Trigger Source field fill the text/image that will open a lightbox in.
In the Lightbox Source field set #my-lightbox-content