Commanders, we extended the current test period of submarines until 04:00 UTC, 31 of October to give you more time and opportunities to try out the new class and its updated mechanics.
In order to enter the test, you need to download the TST client and log in with your account from the main server. More details about joining the test and how to play submarines and against them are available in the article.
Source: WoWS Devblog