Hi! Shinyapps.io does not support the SSL certificate for custom URLs, only http://. There was a statement here in 2018 that this issue is on the backlog: Shiny Apps - Custom URL.
Have there been any new developments? Browsers are getting more and more annoying in directing users away from http:// pages, and the custom URL feature becomes less and less useful. It would be great to have SSL for custom URLs without having to resort to iframe-hacks like described here.