Once in the game navigate to the play online option. And you should see the menu in the picture to the right. Now remember the server name we used when we registered our account? Use that as the “Online ID”. The “Password” is the server password (not your account! password) and your “Account name” is your email you used to register on C&C Online.
Click log in and if we did everything correctly you should see a lobby with other players. If you had any issues you can ask for help on the C&C Online official forums, check the official set up guide or this more detailed steam installation guide.