You can add references to an EndNote Online library in much the same way as you add references to a desktop library.
You can add references manually, you can connect to online databases e.g. PubMed or you can export references from databases.
Some databases give you the option of exporting to EN Online. However for direct export to work in other databases you need to download the plugin for the browser you are using. The plugins (installers) can be found on the EN Online home page.
See also the EN Online - Direct Export