If you have already created a stack with out any stack policy then you can apply stack policy to this stack using AWS CLI only. This can’t be done through Console.
Using Presigned URL, the S3 objects can be accessed securely in PowerBI for limited time.
Generate the content-MD5 value to call S3 API operations (PUT & POST) using POSTMAN
The contents of a file uploaded to S3 bucket can be accessed via Postman using RestAPI.