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.
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Using PowerBI to securely access AWS S3 objects using presigned URLs
Using Presigned URL, the S3 objects can be accessed securely in PowerBI for limited time.
How to generate Content-MD5 value ( base64-encoded 128-bit MD5) of the data
Generate the content-MD5 value to call S3 API operations (PUT & POST) using POSTMAN
How to Get AWS S3 bucket object data using Postman
The contents of a file uploaded to S3 bucket can be accessed via Postman using RestAPI.