Access POST Request body from Custom Authorizer Lambda Function

I've got a custom authorizer lambda function in AWS configured for an API to another Lambda function with a POST resource.

The authorizer is setup as of type Request, not Token, because I am not using tokens in the Authorization header, and instead want to access form data that is being posted.

When I inspect the contents of the event parameter to my authorizer function, I do not see any of the original POST request body (form data). However, if I change my resource to a GET, I see the form data in the queryStringParameters as expected.

(notice below that when the request is POST, the queryStringParameters is always an empty object)

Is there anyway to access the form data from the request in the function, when using a POST method?

Here's an example of what the event parameter to the authorizer function will contain when using POST:

{
  type: 'REQUEST',
  methodArn: 'arn:aws:execute-api:us-east-1:********:********/dev/POST/receive',
  resource: '/receive',
  path: '/sms/receive',
  httpMethod: 'POST',
  headers: {
    Accept: '*/*',
    'CloudFront-Viewer-Country': 'US',
    'CloudFront-Forwarded-Proto': 'https',
    'CloudFront-Is-Tablet-Viewer': 'false',
    'CloudFront-Is-Mobile-Viewer': 'false',
    'User-Agent': 'TwilioProxy/1.1',
    'X-Forwarded-Proto': 'https',
    'CloudFront-Is-SmartTV-Viewer': 'false',
    Host: 'api.myredactedcompany.io',
    'X-Forwarded-Port': '443',
    'X-Amzn-Trace-Id': 'Root=**************',
    Via: '1.1 ***************.cloudfront.net (CloudFront)',
    'Cache-Control': 'max-age=259200',
    'X-Twilio-Signature': '***************************',
    'X-Amz-Cf-Id': '****************************',
    'X-Forwarded-For': '[redacted IP addresses]',
    'Content-Length': '492',
    'CloudFront-Is-Desktop-Viewer': 'true',
    'Content-Type': 'application/x-www-form-urlencoded'
  },
  queryStringParameters: {},
  pathParameters: {},
  stageVariables: {},
  requestContext: {
    path: '/sms/receive',
    accountId: '************',
    resourceId: '*****',
    stage: 'dev',
    requestId: '5458adda-ce2c-11e7-ba08-b7e69bc7c01c',
    identity: {
      cognitoIdentityPoolId: null,
      accountId: null,
      cognitoIdentityId: null,
      caller: null,
      apiKey: '',
      sourceIp: '[redacted IP]',
      accessKey: null,
      cognitoAuthenticationType: null,
      cognitoAuthenticationProvider: null,
      userArn: null,
      userAgent: 'TwilioProxy/1.1',
      user: null
    },
    resourcePath: '/receive',
    httpMethod: 'POST',
    apiId: '*******'
  }
}