AWS Secrets Manager Update Multiple Value with Private Key

I am using the AWS CLI to update the secrets but I am having some issues updating multi-string secrets with a Private SSH Key.

{
    "Name": "multi-value-secret-string-with-ssh-key",
    "VersionId": "red321ds-d3eb-4964-9a8f-7f4ecb375a35",
    "SecretString": {"public": "some-public-key-here", "private": "need-to-insert-the-private-key-here"},
    "VersionStages": [
        "AWSCURRENT"
    ],
    "CreatedDate": 125125123124124312412432,
    "ARN": "arn:aws:secretsmanager:us-east-2:432414312412342141243124:secret:multi-key-XQuwafs"
}

The private key is stored in private.ppk and I want to put the content of that private key file to the private attribute of the secretstring, is this possible via CLI?