I have a feeling this option is missing deliberately to discourage the usage of such improvised separators. You’re kind of working against the strengths of Scrivener’s philosophy (putting those separated chunks in their own documents). Just a user’s theory, though.
In addition, the stock compile formats do recognise an empty line in the editor as a separator request, and will transform it to match the separator design for that format. For example a single hash mark for the manuscript formats, or the little glyph for the Modern format. Nothing is simpler than hitting the enter key twice.
But yes, overall, the idea with Scrivener is that if something is a new section it should probably be a new section in the binder too. Less scrolling, more metadata, more cards to work with, it’s just all around closer to the design than packing lots of ideas, scenes, narratives, topics and so on into longer chunks.
I was looking for a feature which let’s you add the separator in the editor itself. That way it’s easier to copy paste for online publishing like on blogs.
When it comes to such digital publishing, compiling isn’t a part of the process.
Oh okay, I would be using Markdown for something like that (well, I’d use it anyway), because it produces by far the cleanest HTML for pasting into CMS and blog systems. Then you can just use its separator, which is already what you’d like to be using anyway. Three asterisks on a line works, or four hyphens, or 60 hyphens. It’s quite flexible. If the CMS is in the stone age and doesn’t take Markdown text directly, there are plenty of ways to get it converted to HTML.
Of course Scrivener can also convert Markdown to HTML with the compiler, but if for some reason you have a rule about not using the compiler for even basic output, I guess that’s out. Something like the Pandoc browser tool would work fine (don’t worry, it’s offline conversion, that page loads the converter fully into your browser).
If I understand correctly you could simply use <hr> as the separator. That should add a horizontal line when you copy and paste into Wordpress or wherever.