@max thank you for your answer and that would be helpful.

The main idea is to use Auth type=AWS_IAM instead of None.
It was working with OAC but not for POST and PUT requests in my case.
So I have,

• added OAC with reccomendations.
• added a header "x-amz-content-sha256" with signed body hash value like the aws documentation suggest.
• used Pre-request FE side to generate the hash and add is as a header. (tested in PostMan)

const hash = CryptoJS.SHA256(pm.request.body.toString()).toString();
pm.request.headers.add({key: "x-amz-content-sha256", value: hash});