Setting google cloud storage trigger in serverless.yml

For google cloud as provider, how do we set the cloud storage trigger on specific folder within the bucket in serveless.yml. I am only able to set the trigger on bucket by using below code:

handler: objectStoreEventReader
- event:
resource: projects/{projectId}/buckets/{bucketName}"

But how do i set trigger on specific folder rather than a specific folder within the bucket, like i have bucketName “middleware” and we want to set the trigger on middleware/input where input is the folder.

Please, any help is appreciated !!!