Where to specify Amazon S3 bucket directory name to upload file

I am using AWS low level API to upload file to S3 bucket from .net application.

InitiateMultipartUploadRequest initiateRequest = new InitiateMultipartUploadRequest
{
    BucketName = bucketName,
    Key = keyName
};

UploadPartRequest uploadRequest = new UploadPartRequest
{
    BucketName = bucketName,
    Key = keyName,
    UploadId = initResponse.UploadId,
    PartNumber = i,
    PartSize = partSize,
    FilePosition = filePosition,
    FilePath = filePath,
};

Here i specify bucket name but where do i specify directory name within bucket to upload file. eg: Bucket name: SampleBucket

File to be uploaded to folder: Upload/JSON

So here, i need to upload file to JSON folder which is within Upload directory.

Where do i specify this path?