To log onto the test server, you first have to go to the Pirates Online website to register for test access. You have to meet certain requirements like ( currently ) having unlimited access, and having no bans or holds on your account in the past.
Once you register, it may take a few months to even know if you've gained access to test. From there, to log onto test, you have to find the POTCO test
site and download the test version of the game.