Not able to use Try Out on Integrated Developer Portal for Internal Apigee X Instance

[SidDas]

I have an Internal Apigee X Instance (not exposed on the internet) and facing problems in running  "Try This API" on the Integrated Dev Portal. The Integrated Dev Portal is accessible on the internet. 

The following are my settings:

1. Proxy is deployed on an Environment with Host name : prod.apigee.tcs.com and I'm able to invoke the proxy from a VM in the same GCP Project with the following curl command:

curl -i -k -H "Host: prod.apigee.tcs.com" https://xx.xxx.xx.x/tcs-apigee-demo-error-handling/transform-error/500/T05?apikey=xxx

2. I have set a CORS on the proxy with this configuration:

<CORS continueOnError="false" enabled="true" name="CORS-Allow">
    <DisplayName>CORS-Allow</DisplayName>
    <AllowOrigins>{request.header.origin}</AllowOrigins>
    <AllowMethods>GET, PUT, POST, DELETE</AllowMethods>
    <AllowHeaders>{substring(request.header.Access-Control-Request-Headers.values,1,-1)}</AllowHeaders>
    <ExposeHeaders>*</ExposeHeaders>
    <MaxAge>3628800</MaxAge>
    <AllowCredentials>true</AllowCredentials>
    <GeneratePreflightResponse>true</GeneratePreflightResponse>
    <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
</CORS>

3. I have published the proxy with the following OAS spec which specifies the Host in the header

openapi: 3.0.0
info:
  title: Error Handling
  description: Error Handling Framework
  contact:
    email: xx@tcs.com
  license:
    name: TCS Confidential
    url: xxx
  version: 1.0.0
externalDocs:
  description: TCS Apigee Developer Portal
  url: https://xxx.apigee.io/
servers:
  - url: https://xx.xxx.xx.xx/tcs-apigee-demo-error-handling
tags:
  - name: transform-error
    description: Trandform Error Messages
    externalDocs:
      description: Find out more
      url: xx.apigee.io/
  - name: passthru-error
    description: Passthrough backend error meesages
    externalDocs:
      description: Find out more
      url: xx.apigee.io/
security:
  - api_key: []      
paths:
  /transform-error/{error_code}/{backend_id}:
    get:
      tags:
        - transform
      summary: Transform Backend Error Messages
      description: Handle Backend Errors and transform error appropriately
      operationId: transform-error
      parameters:
        - name: Host
          in: header
          description: Host Name
          required: true
          schema:
             type: string
             format: string
             default: prod.apigee.tcs.com
        - name: error_code
          in: path
          description: HTTP Status to be returned
          required: true
          schema:
            type: integer
            format: int64
        - name: backend_id
          in: path
          description: Backend API Target Identifier
          required: true
          schema:
            type: string
            enum: ["T01","T02","T03","T04","T05"]
      responses:
        '200':
          description: successful operation
        '400':
          description: a 400 error occured
        '500':
          description: a 500 error occured
  /passthru-error/{error_code}/{backend_id}:
    get:
      tags:
        - passthru
      summary: Transform Backend Error Messages
      description: Handle Backend Errors and transform error appropriately
      operationId: passthru-error
      parameters:
        - name: Host
          in: header
          description: Host Name
          required: true
          schema:
             type: string
             format: string
             default: prod.apigee.tcs.com
        - name: error_code
          in: path
          description: HTTP Status to be returned
          required: true
          schema:
            type: integer
            format: int64
        - name: backend_id
          in: path
          description: Backend API Target Identifier
          required: true
          schema:
            type: string
            enum: ["T01","T02","T03","T04","T05"]
      responses:
        '200':
          description: successful operation
        '400':
          description: a 400 error occured
        '500':
          description: a 500 error occured
components:
  securitySchemes:
    api_key:
      type: apiKey
      name: apikey
      in: query

When I run this from the Dev Portal, I get the following two errors in the console:

(1) Refused to set unsafe header "Host",

(2) xx.xxx.xx.x/tcs-apigee-demo-error-handling/transform-error/500/T05?apikey=abc Failed to load resource: net::ERR_CONNECTION_CLOSED

Is this because the IP address of the Apigee X instance is not externally exposed? Is there a way to use the Developer Portal "Try Out" on proxy hosted on Internal facing Apigee X instance ?

Thanks in advance !