AWS Lambda(C#)でAPI Gatewayからパラメータを受け取る方法

[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

using Amazon.Lambda.Core;

// Assembly attribute to enable the Lambda function's JSON input to be converted into a .NET class.
[assembly: LambdaSerializer(typeof(Amazon.Lambda.Serialization.SystemTextJson.DefaultLambdaJsonSerializer))]

namespace AWSLambda1
{
    public class Function
    {
        public ResponseParam FunctionHandler(RequestParam input, ILambdaContext context)
        {
            return new ResponseParam
            {
                StatusCode = 200,
                Result = input?.ParamA + " - " + input?.ParamB
            };
        }
    }
    public class RequestParam
    {
        public string ParamA { get; set; }
        public string ParamB { get; set; }
    }
    public class ResponseParam
    {
        public int StatusCode { get; set; }
        public string Result { get; set; }
    }
}

{"ParamA":"ValueA","ParamB","ValueB"}

{
"ParamA": "$input.params('a')",
"ParamB": "$input.params('b')"
}

https://xxxxxxxxxx.execute-api.ap-northeast-1.amazonaws.com/default/AWSLambda1?a=Value1&b=Value2

{"StatusCode":200,"Result":"Value1 - Value2"}