I'd use a different client myself, I've never had good luck using Finder to connect to FTP servers.
Finder used to prompt for username/password when required but you might be able to use one of the following:
This functionality has never seemed to be super well maintained by Apple, so it may not currently work.
Transferring files insecurely is not a justification to permanently install unknown horrors on your Mac
cURL appears to be pre-installed with MacOS with comprehensive support for FTP and other protocols, so I would use that if I need more than the Apple FTP client can deliver. There might even be a cURL GUI for Mac somewhere on the web.