MySQL Workbench - specify Server/User/Pass on command line?
You can use the query parameter and give it the name of one of the connections you have defined (similar for the admin parameter). This will open a SQL IDE tab with that connection.

Instead of a predefined connection you can also use the usual command line params -h127.0.0.1 -uroot -P3306 in conjunction with the query and admin parameters.

For details how this is parsed look in wb_utils_grt.py (connectionFromString(connstr)).

Update:

The documentation says:


  --query [connection|connection_string]:
     Empty: Open a query tab and prompts for a connection.
  
  Connection: Open a named connection.
  
  Connection_string: Create a connection based on the entered connection string, which should be in the form $USER@$HOST:$PORT.