I like <forum.domain.tld> so when I picked my domain name I went for just the name of the site. Without forum or anything else in the domain name itself.
This way I could add other aspects, rather than just a forum, without it seeming odd. So this way I can have wiki.domain.tld or social.domain.tld or blog.domain.tld. (Rather than like wiki.domain-forum.tld or blog.domain-forum.tld.)
When I usually make a forum, I just have ideas to make a forum so I keep it as the TLD root www.the-forum.com instead of a directory or subdomain. If I want to add a portal or something like that in the future, most forum software will do this quite easily without having to manually move anything around.
If it is just the forum that you are holding under that domain, just use the regular domain without the /forum on it. When you get a blog, marketplace or other special page is when you need add the ending.