Reading the stage name in the yaml file

I’m just getting started with serverless and I am new to yaml files in general. I am having some trouble reading the stage name in my yaml file. Here is how I am trying to read it:

        handler: handler.createOrUpdateLookups
            - s3:
                bucket: com.backbar.lookups.${opt:stage}
                event: s3:ObjectCreated:*

When I try to deploy I get this error

A valid option to satisfy the declaration ‘opt:stage’ could not be found.

Question: How do I read the stage name in the yaml file?

Figured it out. It was ${self:provider.stage}