This is all there is to it:
URL is usually localhost.
Database name has to be an existing database on your local postgres-server.
Username and password usually both ‘postgres’ but this will be connecting to your locally running postgres-server where you can set them at you own preference.
Tim’s answer describes how to connect your Mendix app to a local Postgres database so that all of the data for your app is stored in Postgres.
If you want to access external data (i.e. data maintained outside of your Mendix app) from your Mendix app, you can use the Database Connector module. That module can be found here: https://docs.mendix.com/appstore/connectors/database-connector Doucmentation for this module can be accessed here: https://docs.mendix.com/appstore/connectors/database-connector