This page contains examples for the JSON formatted options that can be set for popup forms when using the popup form shortcode or widget. The options for the popup script depends on what popup script is currently loaded on the site, you can check this at Forms → Settings → Tweaks & Troubleshooting → Popup script.
Fancybox (1, 2 & 3)
See the Fancybox documentation for all available options.
See the Magnific Popup documentation for all available options.