To connect your Jellyfin server to Soundiiz, enter your username and password and fill in the URL address with the protocol scheme: "http://" or "https://". So the URL you use on Soundiiz should be, for example: "http://myip:port".
Then, ensure that your Jellyfin network configuration allows remote access and port forwarding if required.
If you're unable to create a playlist with Soundiiz for Jellyfin (you'll see an error message), ensure you can create a playlist yourself in Jellyfin. For example, if you are using Docker, be sure to map the playlist directory too to your composer script.