How to unit test post request to get 403 response

I have a servlet that reads from header.

protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        if (isKeyPass(req.getHeader("externalApiKey"))) {
            resp.sendError(SC_FORBIDDEN, "You do not have permission to access this page.");
            return;
        }
    ....
}

I want to be able to write a unit test to make a post request and get by 403 response. I am using servlet spec 3.0. I saw some response in .net but i am trying to do it in Java. What is the best way to do this.