Suppose we have the following connection string: mongodb://user:password@localhost/test

In this example and according to the documenation, the 'test' database will be used for authentication as well as the database you connect to. The documentation specifies an 'authenticationDatabase' query parameter to customize this a little bit.

For example: mongodb://user:password@localhost/test?authenticationDatabase=admin

This changes the connection string to mean that you should connect to 'test' but authenticate against 'admin'. It would be nice to have GUI support for this.