Hi @olivermorris,
Currently, it is not possible to use an FME Server parameter to reference an S3 Bucket. However, if you want to reference a file directly in a Reader or FeatureReader you can browse your S3 bucket in FME Desktop and select the file. This requires a AWS Web Connection.

Another option would be to use the S3Downloader to first download the file and then read the file with a FeatureReader.
To upload files to S3 they first need to be written to disk and then uploaded with the S3Uploader. There is currently no option to specify an S3 location as the destination for a Writer or FeatureWriter.
Please also check out this related idea and feel free to vote for it or add a specific use case for this.
https://knowledge.safe.com/content/idea/22137/ability-to-mount-s3-bucket-as-a-drive-from-within.html
Best regards,
Gerhard