The easiest way to authenticate to a mongo database is through a username + password authentication.
All the connection information needed to connect must be in a ConnectionString.
If you use MongoDB Atlas, the connection strings for your deployment is provided when you click the Connect button in the Atlas UI, it should have the following format:
mongodb+srv://<username>:<password>@<hostname>
Simply copy paste this connection string in your connector, add the database name and enable the connector. The Connector should connect by itself.