It is rather a very poor user experience to ask him/her to type the url. Why not design this add-on to allow to add it by click on the tab and extract the site info directly?
Also, I noticed that typing the domain like domain.com won't block landing.domain.com. What is the point then? The malicious web site admin can create a second site like landing2.domain.com and go on business as usual.
I really liked to try this add-on at first because I wanted to avoid editing the hosts file directly.
I would appreciate if the developers listed to this feedback.