I would like to create a new one called Demo and add songs in this bookmark. I managed to create the bookmark but I couldn’t figure out how to add one song to that bookmark.
I’d rather use bookmark rather than playlist or tag as I always used bookmarks since I got Roon.
That is not really how bookmarks work. Quoting some text from Dev Mike about Bookmarks from a long ago post (bold emphasis mine):
You can bookmark pretty much any screen in Roon by clicking the bookmark icon in the upper-right, next to Search. Once you’ve saved a bookmark, it will be available from the same drop down, allowing you to quickly return to the screen you saved.
This lets you easily jump to any albums, artists, or playlists you want regular access to, or to return to a browser with a specific focus or sort.
Bookmarks are for screens, sorts of screens, saved focus results, etc. You can create a Tag called Demo, apply it to an album/track, and then create a bookmark on that Tag. Then anytime you add the Demo tag to something it will appear in the Demo bookmark.