Get Raw Body from request

Hey im trying to implement stripe webhooks with lambda functions and i can’t pass stripe signature validation : https://stripe.com/docs/webhooks/signatures
it seems the body is being modified somewhere and the validation fails

if i add a custom request template to get the raw body , serverless remove all other variables and i can’t handle the event with express

functions:
  create:
    handler: posts.create
    events:
      - http:
          method: get
          path: whatever
          integration: lambda
          request:
            template:
              application/json: '{ "rawbody": "$util.escapeJavaScript($input.body)" }'

is there a way to “extend” a template instead of override it ? would like to add the raw body like this :
“rawbody”: “$util.escapeJavaScript($input.body)”,

thanks

You might find it easier to implement using Lambda Proxy integration.