code: public void doSignup( StaplerRequest req, StaplerResponse rsp ) throws IOException, ServletException {
          if (getSecurityRealm().allowsSignup()) {
              req.getView(getSecurityRealm(), "signup.jelly").forward(req, rsp);
              return;
          }
          req.getView(SecurityRealm.class, "signup.jelly").forward(req, rsp);
      }
docs: Sign up for the user account.