np - so the relevant snippet from the flow I sent over is here:

bucket = 'mybucket' # <----- added a constant for no real good reason at all but maybe this would be something that would be meaningful as a param

    @step
    def start(self):
        """Initialise the flow."""
        data = '1,2,3'

        self.path = f's3://{self.bucket}/metaflow-runs/{current.flow_name}/{current.run_id}' # <------ path from current 
        with S3(s3root=self.path) as s3:
            res = json.dumps({'data': data})
            self.url = s3.put('data-test', res) # <-------- we're persisting the whole object path here
            logger.info('Key URL: %s', self.url)
        self.next(self.read_from_batch)