The pregame is played via a Telegram bot who will give you further instructions. So, after you have installed telegram, search for the username @Pandora19_bot, or contact it via this link: http://t.me/Pandora19_bot. You can start the pregame there either by pressing the 'start' button, or if it doesn't appear sending the command /start to the bot. The bot then provides further help and instructions.
Unlike other years, this year you have to authenticate to the pregame. The bot will provide instructions on how to do that. This means that every participant of Pandora can independently do the pregame, you still get points as a team however.