By inserting this to your virtual host you can protect it with a username and password:

<Location />
         AuthName "ProFTPd Administrator"
         AuthType Digest
         AuthDigestDomain /
         AuthDigestProvider file
         AuthUserFile /etc/digest_pw
         Require valid-user
</Location>

Now you just need to provision a digest password file. This example will add the user admin to the realm “ProFTPd Administrator”:

htdigest /etc/digest_pw "ProFTPd Administrator" admin
Adding user admin in realm ProFTPd Administrator
New password: 
Re-type new password:

That's it!