If you need help with your extension, we need to see its code.

Though, there's already Generate HTTP URL Extension, which does exactly what you need.
Just specify the your web hostname in Web server hostname override box.

I'm using WinSCP to manage files on my remote server.
I have an index.html file in my remote directory. I would like to right-click on it and choose an option like "Open in browser" (e.g. Firefox), so it opens the file's public URL directly.

I have already created a WinSCP extension for that.
The URL it opens is:
http://ftp.example.com/txxxxx/xxxx/index.html
(because my session is logged in to ftp.example.com)

But I actually want the URL to be:
http://example.com/txxxxx/xxxx/index.html

I can't figure out how to change this behavior in the extension. Any idea how to fix it?

Thanks!