If using the default posgres user account, I typically go with the following command. In this example, a database named "mydb" is dropped.

sudo -u postgres psql --command "drop database mydb"

If you have created a user account, the -u or --username option and --password flag can be used.

psql --username johndoe --password --command "drop database mydb"

Better yet, you can setup passwordless authentication using the hidden .pgpass file and then connect without the --password flag.

psql --username johndoe --dbname mydb --command "drop database mydb"