In order to train and compete online with other players, you must first create an account on the LFS website, you can read this article and follow the steps to do so.
Once the simulator has been downloaded, you must select the 'Multiplayer' option from the main menu. If you haven't downloaded the app yet, you can do it here.
Then you must click on the second option in the menu 'Join specific host'.
If you have already logged into your account, you must enter the server name and password that was indicated and then 'Join'.
If you have not logged in yet, the following dialog will be displayed:
After clicking on 'OK', you will be asked to enter your account username and password. The data is the one that corresponds to the form you filled out when creating the account.
Once the data has been entered, you can click on 'Unlock' on the lower right corner.
Then you must again click on the second option in the menu 'Join specific host'.
Finally, you must enter the server name and password that was indicated.
Once on the server, you must click on 'Join Server' and you will be able to join the next race.