How to use IAM role to access S3 bucket

I want to upload a file to S3 bucket, but my company want to use IAM role as opposed to access keys.  This is AWS documentation on how to upload to S3 asynchronously:

S3AsyncClient client = S3AsyncClient.create();
		CompletableFuture<PutObjectResponse> future = client.putObject(
				PutObjectRequest.builder().bucket(BUCKET)
						.key(fileName)
						.build(),
				AsyncRequestBody
						.fromFile(fromFile.toPath()));
		future.whenComplete((resp, err) -> {
			try {
				if (resp != null) {
					System.out.println("my response: " + resp);
				} else {
					// Handle error
					err.printStackTrace();
				}
			} finally {
				// Lets the application shut down. Only close the client when
				// you are completely done with it.
				client.close();
				
			}
		});

		future.join();
	}

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I don't see anywhere to put in IAM role info.  I tried to put it in ~/.aws/credentials in this form:

[useraccount]
aws_access_key_id=<key>
aws_secret_access_key=<secret>

[somerole]
role_arn=<the ARN of the role you want to assume>
source_profile=useraccount

but so far haven't gotten it to work.  I read somewhere you need to use STSAssumeRoleSessionCredentialsProvider but didn't see any good examples.  My main question is do I even need to do anything if I already assigned the IAM role to an ECS instance.  Can someone help me?   Thanks.