The following command creates a multipart upload in the bucket my-bucket with the key multipart/01:
aws s3api create-multipart-upload --bucket my-bucket --key 'multipart/01'
Output:
{
"Bucket": "my-bucket",
"UploadId": "dfRtDYU0WWCCcH43C3WFbkRONycyCpTJJvxu2i5GYkZljF.Yxwh6XG7WfS2vC4to6HiV6Yjlx.cph0gtNBtJ8P3URCSbB7rjxI5iEwVDmgaXZOGgkk5nVTW16HOQ5l0R",
"Key": "multipart/01"
}
The completed file will be named 01 in a folder called multipart in the bucket my-bucket. Save the upload ID, key and bucket name for use with the upload-part command.