The criteria in the condition rule does not have to evaluate to true. For example, a condition rule such as "current page url = abc" will never be true, but it will make the lightbox eligible for display and it will be possible to trigger the lightbox via the API. While a lightbox triggered via an API call requires a condition to be set in order to be eligible for display, the logic that that determines how and when the lightbox is displayed is entirely set in the Custom JS.
The following Web Forms API call will launch a lightbox.
This code should be placed in Custom JS (Parent).
The GUID value for a lighbox can be found by clicking on the ID of a lightbox.