Every site implementation is different...
If you are using the Adobe Client Data Layer, that your developers are pushing events (with relevant data) to that data layer, when the lightbox is opened, you would get your developers to push that information to the data layer... then you would create a trigger for that event in Launch, making sure that all the relevant data and the correct type of event is sent (i.e. a page view or an action).
If you are using something else, like maybe you are using the GTM data layer so that you can leverage both Adobe and GA in parallel... it would be similar to the above.
If you are using a custom solution, then you might have your developers trigger a custom JS event, and you create a listener for that.... or maybe something else that falls in line with your site design....
Without knowing how your site and the tracking implementation was done, it's hard for us to recommend an exact course of action...