ftp://username@hostname/Assuming a password is required, your browser will then prompt you for one.
It is even possible, but inadvisable, to put a password in a URL:
ftp://username:password@hostname/This is unadvisable for several reasons. The URL being opened may be determinable by other users on the same machine on which you are browsing (as from a command line). The URL retrieved from the remote machine may be logged in some non-secure place on the remote machine. Your browser history would then also contain a copy of your password.
Microsoft is disabling support in Internet Explorer for this kind of information inclusion in HTTP and HTTPS protocol URLs.