How to return some json to specific path when www.mydomain.com hosted in S3?

Hi, @Asit Dixit

If you have "The specified key does not exist" error, you need to create an S3 bucket with a custom domain name.
I was able to create and access an S3 bucket as follows.

% curl http://hoge1224.tak1wa.com/.well-known/assetlinks.json
{
    "hoge":"fuga"
}

Note: I'm assuming you don't use CloudFront. If you use CloudFront, you can create an S3 bucket without using a custom domain name.