Serverless: Configuration warning at '[0]': unsupported function event

I am attempting to attach an event trigger to a lambda function using an existing s3 bucket using an existings3 serverless plugin. Documentation for the plugin can be found here:

The event giving me problems goes as follows:

    handler: src/functions/forward/index.handler
    name: ${self:custom.apiVersion}-${}-email-forwarding
    memorySize: 512 
      - existingS3:
          bucket: ReportsForwardingBucket
            - s3:ObjectCreated:*

Not sure what is causing the problem I saw a lot of people saying their indentation or spacing was causing the issue but I double checked multiple times. Any help would be appreciated.