Note that you can avoid the need to even require users to accept third-party cookies, at least for YouTube, by using www(dot)youtube-nocookie(dot)com instead of the regular version without -nocookie, in the URL. I am not sure if the IB software will automatically rewrite these to the cookied version though. Although since the forum software automatically detects YouTube links and rewrites them into embeds, it would be incrementally trivial to use the nocookie version instead.

This is an officially supported domain by YouTube, which you can verify by clicking on any YouTube video, clicking Share, choosing Embed, then scroll down to the checkboxes and check "Enhanced Privacy Mode" -- you will see the link in the embed code change from youtube to youtube-nocookie.

(My in-development-and-may-never-be-published userscript for FT does this conversion automatically.)