Hello,
mongo shell syntax:
studio 3T.exe mongodb:// --username --password --database --port
Something like that would be appreciated.
Thank you for your time and attention to this matter.
Welcome to the 3T Community! 
Unfortunately, as of right now, that’s not supported. However, depending on your use case, there may be a workaround.
If there’s just one connection (or a set of connections) you would always like to be connected to when Studio 3T starts, you could enable Session Restore, which would restore all previously connected-to connections on startup.
Alternatively there are also our Quick Connect shortcuts: In the Connect dialog, right click on a connection (or a set of connections) and then assign one of the nine Quick Connect shortcuts. You can then hit Alt + 1-9 to quickly connect to your chosen connections from anywhere within the application.
We would also like the ability to create a connection via the command line. In our case, our credentials rotate daily, so any and all saved connections become invalid within 24h. This takes away the utility and usefulness of the quick connect and session restore features.
We often need to connect to multiple databases daily, so the ability to create an integration with our credential provider to directly open and connect to a database would be a great benefit. We have done this with the other DB GUIs we use and find that workflow very enjoyable.