...
In the Authorization tab select Type “AWS Signature” and enter your AccessKey and SecretKey, which you get when you create an S3 token in Content Portal. Under Advanced you must enter “S3” s3” (lowercase S3!) for the Service Name.
In the Body tab choose raw and choose XML in the Text dropdown. Enter a body like:
...
You must also add the Key Content-MD5 with the value {{contentMD5}} (that is Postman variable syntax) in the Headers tab. With recent Postman 10.21.0 it seems this reports contentMD5 is an Unresolved Variable but V4 signature still works probably because X-Amz-Content-Sha256 is used.
Finally you must enter this in the Pre-request Script tab so that {{contentMD5}} variable is available:
...