Dynamically set header values in AWS API Gateway

Question

Question

What are the necessary steps to dynamically set headers in the response from API gateway using the context from the authorizer response.

Current Approach

Here are the current steps I have implemented to set the response header(s) dynamically.

1. Dynamically generate the response header value in the authorizer-lambda and return it in the context of the authorizer response.

return {
    principalId,
    policyDocument: {
      Version: "2012-10-17",
      Statement: [
        {
            Action: "execute-api:Invoke",
            Effect: effect,
            Resource: this.resource(this.event.path),
        },
      ],
    },
    context: {
      dynamicValue: dynamicValue
    },
};

2. Configure the gateway response headers. (NOTE: This is where I an running into issues.)
   • API Gateway → [Choose Gateway] → Gateway Responses → Unauthorized
   • Add Response Header:
     • Response Header: x-new-header-value
     • Value: $context.authorizer.dynamicValue

Problem(s)

When using the mapping template for the value in the step above I get the error below. This makes me think I am missing a configuration step.

Invalid mapping expression specified: Validation Result: warnings : [], errors : [Invalid mapping expression specified: $context.authorizer.doLogout]