Users should have access to their home directory–so the domain owner user should have access to cgi-bin, public_html, etc, and so on, within their home. Mailbox users will generally only have mail in their home directory–and so, I assume you’re logging in as a mail user.
You can create additional website maintenance users which do have FTP access to the domain home directory, but normal mailbox users don’t get that level of access and I don’t believe they should.
The domain user has FTP access to all folders within the domain home (i.e. /home/domain) and not just public_html, while mail users have access to their own home directory.
You can also create a "web content administrator" user that has access FTP access to the domain folders, but no administrative access via Virtualmin/Webmin.
But I’m not sure that’s what you’re after. What folder do you want to assign an FTP user to? We’re limited to what the FTP server (ProFTPd) will let us do, so if the FTP server can do it, we can add support for it–but if the FTP server doesn’t support it, there’s not much we can do about it.
DAV is more flexible, in that very fine-grained access controls are possible without any file system permissions questions coming up (usually). There are quite a few aspects of this which haven’t been integrated into Virtualmins support for DAV, and it’s not really very easy to abstract into a GUI…
Anyway, if you can be more specific about what you’re trying to do, I might be able to tell you how (or at least confirm that it isn’t possible with FTP).
You can create a user that has this as his home directory. You almost certainly shouldn’t set it up for mail and other features–just make it exclusively for use in administering the forum website content. Allowing mail and other features might confuse whatever software you’ve got running in that directory.