Not sure what about my suggestion would be wrong. Just use a PHP proxy instead:Changed the link to contain "https". The issue has been solved.
Unfortunatetly, we can't do many things to prevent that, only change the links manually.
What if the service doesnt have SSL certificate at that endpoint? Browser would just refuse the connectionOr you could set up an event listener on the client-side to check the post when the user submits to replace every occurrence of http:// to https://
The same can be done and necessary on the backend as well.
And you can always educate the users by informing them to use the proper protocol whenever they try to post http.
Although, some broken links will surely be present when someone tries to share some shitty old site.
In the past, we only accepted imgur links and forced them to https. That worked quite well but the downside is everyone has to know to upload everything to Imgur unless we scraped image links, placed them to Imgur, then replaced image urls.Browser would just refuse the connection