This assumes you have already configured the aws command line tool. If not, check out my article on Getting Started with the AWS CLI.

The aws secretsmanager create-secret command can be used to create a secret.

aws secretsmanager create-secret \
--name postgres \
--description "postgres username and password" \
--secret-string "{\"username\":\"john.doe\",\"password\":\"itsasecret\"}"

Something like this should be returned.

{
    "ARN": "arn:aws:secretsmanager:us-east-1:123456789012:secret:postgres-tA1FKB",
    "Name": "postgres",
    "VersionId": "0a8aa38f-b95e-42ab-acc9-7745aa710f23"
}