I’m not sure will they allow it or not, but your conversion rate will surely be better if you go with a domain name. It just looks more professional and potential customers will have more trust.
You don’t technically need one, no. If you do buy one (it can result in higher trust and conv. rate, potentially) make sure you connect it directly via clickfunnels (assuming this is an option) rather than routing users through a domain level redirect. For redirects, FB will ignore the first URL and will show the final URL in the ad itself.
I would personally get a domain, and as @Adnan mentioned, they’re super cheap so there’s no real reason not to.