Four or five days ago, I created a forum. I'm the only member. My website is not listed in Google, Yahoo, or Bing. I can't find any place to post a link to it.
Getting people to access your website/forum can be hard when the marketing is low for that website or the SEO optimization is low or does not even exist. In this case, I would like to recommend you to use a directory submitter software to enlist your website so that Google, Yahoo or even Bing will find your website. Nowadays, forum scripts have embedded support for addons which could improve your SEO ranking.
Post links in relevant category on Quora.