In the Google Ads online interface, lightbox ads are available for the campaign sub-type called "Display Network only - Engagement."
Instead of a campaign sub-type, Google Ads Editor has an ad group-level setting called "Ad group type." This setting on the Ad groups tab has two options: Default and Engagement.
The Ad group type setting is only editable for new, unposted ad groups. To specify a newly created or pasted ad group as an engagement ad group, choose Ad group type > Engagement.
Once an ad group has been posted to Google Ads, it isn't possible to change the ad group type.