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MediaWiki: creating a new user in private wiki

I've installed the MediaWiki engine hoping to create a private wiki with only two users, myself as an administrator to post information and another user to read it. Nobody warned me about this, but it turned out to be much complicated than I could imagined. I have to use jSON requests to perform actions like creating users and I have absolutely no experience in that. I managed to form a proper request, but now it tells me to use the POST request. I even found this site: http://requestmaker.com/ to simplify things, but still have no idea how exacly do I submit this command. Can I please have a newbie explanation on how I do this?