how can I use proxy+ to handle the root resource on api gateway to return body?

API Gateway with proxy+ works fine for me, the only thing is that the root path always return "Missing Authentication Token", so I found I should setup "ANY" method for the root path, but it would return the response with header in json.

{
  "statusCode": 200,
  "body": "{\"message\":\"Welcome!\"}",
  "headers": {
    "x-powered-by": "Express",
    "access-control-allow-origin": "*",
    "content-type": "application/json; charset=utf-8",
    "content-length": "22",
    "etag": "W/\"16-XP9xOOkLTTczHdpNV2DV26X8ykM\"",
    "date": "Tue, 03 Mar 2020 01:15:05 GMT",
    "connection": "close"
  },
  "isBase64Encoded": false
}

how can I get return only the body?