Hi Harshraj,
It's not possible to get a List of only buckets from a single region. The only way to get a list from a single region is to call ListBuckets and to filter the output afterwards.
This is also not possible within AWS itself (without Mendix): https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListBuckets.html
The reason the Region is being used as an input in Mendix is because we use it for creating the "S3ClientBuilder" in the Java code.