AWS Lambda parameter passing error | API Gateway

Question

I am working with a simple AWS Lambda function :

public class CreateOrderLambda {

    private final ObjectMapper objectMapper = new ObjectMapper();
    // This Particular Order needs to be saved to the DynamoDB.

    private final DynamoDB dynamoDB = new DynamoDB(AmazonDynamoDBClientBuilder.defaultClient());

    public APIGatewayProxyResponseEvent createOrder(final APIGatewayProxyRequestEvent input,
                                                    final Context context)
            throws JsonProcessingException {

        Order thisOrder = objectMapper.readValue(input.getBody(), Order.class);

Now, Here is how the template looks like :-

  CreateOrdersFunction:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabspw/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      CodeUri: ordersapi
      Handler: com.aditya.learn.function.CreateOrderLambda::createOrder
      Policies:
        - DynamoDBCrudPolicy:
            TableName: !Ref OrdersTable
      Events:
        OrderEvents:
          Type: Api
          Properties:
            Path: /orders
            Method: POST

While I hit the aforesaid API through API Gateway, I am able to execute the request well :-

curl --location --request POST 'https://o2cf5ti7ik.execute-api.us-east-1.amazonaws.com/Prod/orders/' \
--header 'Content-Type: application/json' \
--data-raw '{
    "id":1001,
    "itemName":"Nariyal",
    "quantity":3
}'

But, while testing the Lambda standalone from console with following request :

{
  "id": 1004,
  "itemName": "HoneyAlmondFlakes",
  "quantity": 7
}

It gives following error :-

START RequestId: 63aa8edd-38b3-408e-8727-652393964f14 Version: $LATEST
argument "content" is null: java.lang.IllegalArgumentException
java.lang.IllegalArgumentException: argument "content" is null
    at com.fasterxml.jackson.databind.ObjectMapper._assertNotNull(ObjectMapper.java:4757)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3515)
    at com.aditya.learn.function.CreateOrderLambda.createOrder(CreateOrderLambda.java:25)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)

END RequestId: 63aa8edd-38b3-408e-8727-652393964f14
REPORT RequestId: 63aa8edd-38b3-408e-8727-652393964f14  Duration: 4.72 ms   Billed Duration: 5 ms   Memory Size: 512 MB Max Memory Used: 134 MB 

Basically, it's not able to typecast the request into the custom object.

Is the API0Gateway passing the request to Lambda in some special way ? What am I missing in testing the Lambda standalone ?

Any help shall be highly appreciated.

Basis of Marcin's response :-

Tried with following structure of event :-

{
      "resource": "/",
      "path": "/",
      "httpMethod": "POST",
      "requestContext": {
          "id":7,
          "itemName":"Flakes",
          "quantity":6,
          "resourcePath": "/",
          "httpMethod": "POST",
          "path": "/Prod/",
          "requestId": "JKJaXmPLvHcESHA=",
          "time": "23/Jan/2021:05:16:23 +0000"
      },
      "headers": {
          "accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9",
          "accept-encoding": "gzip, deflate, br",
          "Host": "70ixmpl4fl.execute-api.us-east-2.amazonaws.com",
          "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.132 Safari/537.36",
          "X-Amzn-Trace-Id": "Root=1-5e66d96f-7491f09xmpl79d18acf3d050"
      },
      "multiValueHeaders": {
          "accept": [
              "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9"
          ],
          "accept-encoding": [
              "gzip, deflate, br"
          ]
      },
    "isBase64Encoded": false
  }

Still same error :-

START RequestId: 028d4942-f2cf-4dbc-afe3-08783a106555 Version: $LATEST
argument "content" is null: java.lang.IllegalArgumentException
java.lang.IllegalArgumentException: argument "content" is null
    at com.fasterxml.jackson.databind.ObjectMapper._assertNotNull(ObjectMapper.java:4757)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3515)
    at com.aditya.learn.function.CreateOrderLambda.createOrder(CreateOrderLambda.java:25)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)

END RequestId: 028d4942-f2cf-4dbc-afe3-08783a106555
REPORT RequestId: 028d4942-f2cf-4dbc-afe3-08783a106555  Duration: 371.97 ms Billed Duration: 372 ms Memory Size: 512 MB Max Memory Used: 134 MB Init Duration: 2568.48 ms