The correct answer here is to consult with a lawyer licenced to practice in your jurisdiction (or whichever jurisdiction will be the relevant one for the forum’s legal operation) who specialises in online copyright matters. Only they’ll be able to confirm exactly how it needs to be done to have the full protection of the law.
However, if that is beyond your resources, I’d suggest taking a look at exactly how Stack Exchange does it. They have a policy that all user-contributed content is CC-BY-SA. I would presume that, at some point, competent legal advice was sought to ensure that they were doing things in a legally defensible way. Certainly, given how much VC they’ve taken, I’d expect that at least one investor raised it as a point of due diligence.
As far as I can tell, they only mention the forced content licencing in the page footer, and in their ToS. Specifically, section 3 of the Terms of Service simply says,
You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license. You grant Stack Exchange the perpetual and irrevocable right and license to [blah blah blah]
It’s been a while since I signed up to an SE site, but from memory there’s no explicit checkbox anywhere saying “I have read and agree to abide by the ToS”. Presumably, their legal advice was something along the lines of, “since every site has some sort of ToS, it is reasonably expected that a user is aware of the existence of a ToS and has familiarised themselves with it proactively”. Which is, of course, exactly the sort of deranged lawyer-land horse excrement that makes lawyers rich… nobody reads ToSes. It takes a month a year to read all the ToSes the average person purportedly agrees to…
Sorry, got sidetracked there. Anyway, if you follow SE’s lead, you can put something in your ToS, and something in the footer (IMO, something in the editor would be better). But, of course, this isn’t legal advice, I Am Not A Lawyer, I only play one on the Internet, etc etc etc. Please read my Terms of Service for more details.