How to automate AWS Elastic Transcoder Jobs for s3 buckets

Question

What I want: To add watermarks to all video files that are uploaded to the S3 bucket (mov, mp4, etc.). Then overwrite the file with its same name with the newly transcoded file that has the watermark on it.

So, I was able to manually do this by creating a pipeline and job with an elastic transcoder, but this is manual. I want this done the moment a file is uploaded to the server, overwrite the file with the new file and boom.

One, this should be a feature already but not sure why it is site.

And two, How can I have this automatically done? Any advice? I know it is possible just not sure exactly where to start here