serverless.yml

functions:
  hello-world:
    handler: hello-world.handler

stepFunctions:
  stateMachines:
    myStateMachine:
      definition:
        StartAt: firstTask
        States:
          firstTask:
            Type: Task
            Resource:
              Fn::GetAtt: [hello-world, Arn]
            Next: secondTask
          secondTask:
            Type: Task
            Resource: arn:aws:states:::lambda:invoke
            Parameters:
              FunctionName:
                # you can use Ref to get the function name
                Ref: hello-world
              Payload:
                answer: 42
            Next: thirdTask
          thirdTask:
            Type: Task
            Resource: arn:aws:states:::lambda:invoke.waitForTaskToken
            Parameters:
              FunctionName:
                # you can also use Fn::GetAtt to get the ARN
                Fn::GetAtt: [hello-world, Arn]
              Payload:
                token.$: $$.Task.Token
            End: true