type = s3
provider = Other
access_key_id = VerySecret
secret_access_key = EvenMoarSecreter
endpoint = https://filesync-server-1234.example.com:12345/
Run rclone config file to show where the rclone config file is and edit that file putting the above in.
Or alternatively run through rclone config using the interactive wizard. Create an s3 remote with provider Other. It will prompt you for the above info. If you don't know the answer to any of the questions just press enter to take the default.
Then try rclone lsf myservice: and hopefully you'll see buckets you recognize.