FTP settings for subdomain?

Can someone please guide me in entering settings for a subdomain
account? If I enter in /public_html/subdomain_name for the Host
Directory, and enter my main username/password, click on the test
button and it connects successfully. If I create an ftp account with
the subdomain name and give it a password, and then try using that for
my username/password, but I get a 30 authentication error when I run
the test.

Any advice?