AppSync to SNS with Serverless Framework

datasource.yml

  resolvers:
    Mutation.doSomething:
      kind: UNIT
      request: resolvers/sendMessage.request.vtl
      response: resolvers/sendMessage.response.vtl
      substitutions:
        topicArn: !GetAtt MyTopic.TopicArn
      dataSource:
        type: HTTP
        config:
          endpoint: 'https://sns.us-east-1.amazonaws.com/'
          iamRoleStatements:
            - Effect: Allow
              Action:
                - sns:Publish
              Resource: !GetAtt MyTopic.TopicArn
          authorizationConfig:
            authorizationType: AWS_IAM
            awsIamConfig:
              signingServiceName: sns
              signingRegion: !Ref AWS::Region

request.vtl

#set($message=$util.urlEncode($util.toJson($ctx.args.post)))
{
  "version": "2018-05-29",
  "method": "POST",
  "resourcePath": "/",
  "params": {
    "headers": {
      "content-type": "application/x-www-form-urlencoded; charset=utf-8"
    },
    "body": "Action=Publish&Version=2010-03-31&TopicArn=${topicArn}&Message=${message}"
  }
}