Google App Engine server not adding CORS headers - node.js

Question

I have an Express server as backend of my Dialogflow chatbot, when I access from the same host, I have no problem requesting the API, but when I access from the Firebase hosting whereI have my web App, I cannot request anything since the CORS headers are not being added, even though I added them.
This is the code:
// Sends static files from the public path directory
app.use(express.static(path.join(__dirname, '/Public')))
// Set Server Config
app.use(bodyParse.urlencoded({
extended: true
}));
app.use(bodyParse.json());
app.use(helmet.frameguard());
app.use(function (request, response, next) {
// Website you wish to allow to connect
// Request methods you wish to allow
response.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');
// Request headers you wish to allow
response.setHeader('Access-Control-Allow-Headers', 'X-Requested-With,content-type');
response.setHeader("Access-Control-Allow-Origin", 'https://newagent-249c5.web.app/');
// Pass to next layer of middleware
next();
});
// Configure Routes
app.use('/api', apiRoutes);
// Server index.html page when request to the root is made
app.get('/', function (request, response, next) {
response.sendFile('./Public/index.html');
});
var server = app.listen(8080, function () {
var host = server.address().address;
var port = server.address().port;
console.log("server listening at http://%s:%s", host, port);
});
This is cross-origin
This is the headers when I access from the same origin

Answer 1

For CORS most important is Access-Control-Allow-Origin header, and that client, who requesting, has host equal to value of this header. You can test things out pretty simple locally, spin up simple express server on, say, port 3000, and serve index.html with another server, express or nginx, on some other port. Then add request in index.html to first server, and try to fix CORS on this simple example.