You need to include http://... in the server URL.
No, the username and password should be the one you use for logging into your Jellyfin server, not Windows.
I am not sure if http://DELL-DESKTOP:8096 is reachable outside of Windows environment. I have not touched Windows in quite a while, with Linux being my daily driver. Have you tried connecting to this URL with the Jellyfin mobile app? I would also suggest that you simply enter the IP address of your server, e.g. http://192.168.x.x:8096 as IP addresses should be reachable within the same network unless Windows blocked the requests.