1. S3 Bucket Policy

Make sure to replace the Account Number and S3 Bucket Name.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::ACCOUNT-B-NUMBER:root"
            },
            "Action": [
                "s3:GetObject",
                "s3:PutObject",
                "s3:PutObjectAcl"
            ],
            "Resource": [
                "arn:aws:s3:::mykplabs-central/*"
            ]
        }
    ]
}

2. IAM Policy for IAM User of Account B

Make sure to replace the S3 Bucket Name.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:GetObject",
                "s3:PutObject",
                "s3:PutObjectAcl"
            ],
            "Resource": "arn:aws:s3:::mykplabs-central/*"

        }
    ]
}