For the practice servers, use the server name and password in the server-information channel for your series on Discord to find and connect to the server using the in-game multiplayer menu.
For the race servers, we provide 2 links in the server-information channel on Discord: one for use with the game closed and one for use with the game opened. We recommend using the "Game Closed" link, unless you use VR. If you race in VR, start everything normally until you're on the main menu in-game, then use the "Game Opened" link to connect to the server. Ignore the browser window that pops up, but go back into the game and you'll see the prompt to either "Drive" or "Spectate". Click the appropriate option, if you clicked "Drive", select your car and you will load into the server as usual.