BookMarkable is for bookmarking anything (with the right plugin). This is how you write your own plugin.
You can either use Plugin Registration dialog to create new plugin from a template. The dialog will create a temporary, read-only file that will have to be saved or copied into the Scripts folder. This is quite important, otherwise MacOS may refuse to run the script.
Downloading and modifying one of the sample plugins is good way to start start as well.
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The scripts folder is under your home folder Library/Application Scripts/com.onflapp.BookMarkable
Use BookMarkable’s Help menu or the Plugin Registration dialog to go to it.
If you want to use Finder directly: