You will have to see which platform they use. e.g. if it is a forum, then you should register there and put the link in your sig with anchor text.
before trying to do any posts/comments, it makes sense to check if the existing posts/comments have any do-follow links in them
In my case, those .edu or .gov link are using blog platform. And every page of it is an article. You can leave your comment about the article after reading it. You must leave your relevance comment to get approved...
EDU. & GOV. sites are too good sites for us. So check out that if you can register in that site then try to add your site link through profile, blog, forum post etc..