How to specify a c++ lambda function with provided deployment package

I have followed the guide in https://aws.amazon.com/blogs/compute/introducing-the-c-lambda-runtime/ to package my c++ lambda function into a zip file. The zip file has the contents:
function.zip
bin
function
lib
many .so files, etc.
bootstrap

In the guide the lambda is deployed using the aws lambda create-function with the handler specified to be the same as the zip file name.

I would like to configure this lambda function in my serverless.yml and deploy it with the rest of my lambdas. I am not sure what the handler field below should be and using the zip file name didn’t work out.

functions:  
  function:
    runtime: provided
    package:
      include: 
        - function.zip
    handler: function 
    events:
      - http:
          path: function
          method: get
          cors: true
          authorizer: aws_iam

When I run serverless deploy -v I get warnings like:

Serverless: WARNING: Entry for function@function could not be
retrieved. Please check your service config if you want to use
lib.entries.