You can enter a server address manually by enabling Advanced mode at the bottom right of the web interface and entering it at the Server address field on the Connectors page, Homebrew/MMDVM settings. Make sure the Homebrew/MMDVM connector is in MMDVM protocol mode for FreeDMR servers.
Do you know an always updated official source of FreeDMR server list in computer parseable format (JSON/XML/CSV etc.)? We'd like to add it as a source for our server list database.