Do forums accept YouTube URLs in their signature? Most forums do not accept I believe. Are there a few that accept them? Is it beneficial for the Internet marketer to rely on forum signature campaign to advertise his or her YouTube creations? What is your experience in this regard?
A serious Internet marketer wouldn't bother with that. Most forum signatures are no-follow. Forums aren't for advertising, SEO and all that. Certainly if you make a good contribution to a forum, people might take an interest in your signature, but it's not going to make you a living or get you a lot of exposure. It seems like 99+% of marketing/SEO advice out there is sleazy, dodgy nonsense, so be very careful what you take to heart.
Why do you need a YouTube url in your signature? You can simply embed your video in a blog page and put your blog page url in your signature in almost any forum that accepts signatures. Forum marketing can be very profitable but it depends on 1) your marketing niche 2) quality of forums you market on 3) types and quality of content you provide in forum posts 4) the ability to make thread titles that convert forum users to your thread readers 5) ability to make highly converting signatures 6) schedule of your forum activity 7) proper tracking and analyzing results. For best results you need a well-thought forum marketing system to follow. It must be professional.
Without promoting your youtube videos through forum, try some other ways and that would be great. Because, that will help you in seo and also provide you visitors. You can search on google to get the list of promoting your youtube url.