There are a few reports in a Bing Forum thread that adding a site to Bing Webmaster Tools might not work. Specifically, if the URL or domain contains a hyphen (dash) such as www.best-domain.com.
Brett Yount from the Bing Webmaster team confirmed the bug, saying:
Currently, we are having a few difficulties which I just received confirmation from the indexing team. They are currently working on it, but said that if you try a couple times, it should work. If not, and your site isn't in the index at all , please post on the not in index thread and I will work to get your home page (only) into the index.
I personally tried adding a domain with a hyphen and it worked for me on the first try. So maybe it is resolved or maybe those specific domains have other issues?
Forum discussion at Bing Forum.
