How to access custom header from AWS Lambda Authorizer? - aws-api-gateway

Question

I have created an Authorizer in AWS API Gateway. This Authorizer refers to a Lambda Function.
I am passing the following values in header, to the API Endpoint using Postman.
{
"type":"TOKEN",
"authorizationToken": "testing2",
"methodArn": "arn:aws:execute-api:us-west-2:444456789012:ymy8tbxw7b/*/GET/"
}
The above header values are received in the Lambda Function. I can see this through the logs in CloudWatch.
I want to pass additional value 'clientID' in the header. So I pass the following values in the header from postman.
{
"type":"TOKEN",
"authorizationToken": "testing2",
"methodArn": "arn:aws:execute-api:us-west-2:123456789012:ymy8tbxw7b/*/GET/",
"clientID" : "1000"
}
In this case, the Lambda function does not get the clientID. I checked various threads in SO, and understood that this can be achieved mapping header. So I did the following.
In the "Method Execution" section of the API method, I created a new header clientID. In the "Integration Request" section, under "HTTP Headers" section I provided the following value
Name: clientID
Mapped from: method.request.header.clientID
After doing the above, I deployed the API and tried to call the method from Postman, but the clientID is shown undefined. Following is the code that I have written in Lambda Function
exports.handler = function(event, context, callback) {
var clientid = event.clientID;
//I always get event.clientID undefined
console.log("The client ID is:" + event.clientID);
}
EDIT
Following is the error from the CloudWatch Log.
START RequestId: 274c6574-dea5-4009-b777-a929f84b9a9d Version: $LATEST
2019-09-19T09:40:25.944Z 274c6574-dea5-4009-b777-a929f84b9a9d INFO The client ID is:undefined
2019-09-19T09:40:25.968Z 274c6574-dea5-4009-b777-a929f84b9a9d ERROR Invoke Error
{
"errorType": "Error",
"errorMessage": "Unauthorized",
"stack": [
"Error: Unauthorized",
" at _homogeneousError (/var/runtime/CallbackContext.js:13:12)",
" at postError (/var/runtime/CallbackContext.js:30:51)",
" at callback (/var/runtime/CallbackContext.js:42:7)",
" at /var/runtime/CallbackContext.js:105:16",
" at Runtime.exports.handler (/var/task/index.js:40:4)",
" at Runtime.handleOnce (/var/runtime/Runtime.js:66:25)",
" at process._tickCallback (internal/process/next_tick.js:68:7)"
]
}

Answer 1

I have understood why I was not getting the value in the header. I have done the following
1) Instead of type TOKEN, I used type REQUEST in the header. I understood this by reading the following link. This link also contains code for Request type.
https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-use-lambda-authorizer.html
2) I removed all the mapping from Method Request and Integration Request.
3) Deployed the API.