In the link below it's specified in the docs that we can create secrets from a JSON file containing the values of the secrets.

aws secretsmanager create-secret --name MyTestDatabaseSecret \
    --description "My test database secret created with the CLI" \
    --secret-string file://mycreds.json

Unfortunately, I don't find a way to do this with aws cdk. The create-secret method doesn't accept values of the secrets themselves and code like below will autogenerate a secret.

    new secretsmanager.Secret(this, 'Secret', {
      description: 'secret description,
      secretName: 'secretName'
    });

Secrets manager docs from CLI:
https://docs.aws.amazon.com/cli/latest/reference/secretsmanager/create-secret.html#examples

AWS CDK secrets manager docs:
https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-secretsmanager.Secret.html