Create a MySQL, Microsoft SQL, PostgreSQL or Amazon Aurora database from the menu DevOps > Database > RDS > +Add button above the table.
Connect to the database
Once the database is created which takes about 5 to 10 mins then you can expand the created database to fetch the endpoint and credentials. Use this endpoint and credentials to connect to the database from your application running in the EC2 instance. Note that the database is only accessible from inside the EC2 instance (including the containers running within it) in the current tenant.
The best way to pass DB endpoint, name and credentials to your application for security purposes is through environment variables (ENV) of the Service.