In order to create an HTML lightbox, please follow the steps below:

Step 1: 

Go to Dashboard >> Pages >> Edit >> Create two nested columns

Step 2: 

Set the class of the outer column to hidden. Click on Save Element button.

Step 3:

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. 

Step 4:

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