I bought the game but can't play multiplayer cos it ask for CD-Key which I didn't get. And I even can't add screen...

Go to massgate.org for help setting up the multiplayer. Also you can ask questions on the discord channel https://discord.gg/sp9NmbG

Also a great idea to join the discord because that is how you will know when games are on

for a CD key and to be able to play online you need to go to this web page, here you can create a key, follow the instructions to play online

http://www.massgate.org/multiplayer.php#gog_key