How Do I post APN??

I want to send APN to update a pass in the Wallet app. Followed this guideline. https://developer.apple.com/documentation/walletpasses/adding_a_web_service_to_update_passes#3733254

I tried with Typescript (Firebase Functions) like this.

            var path = "/3/device/"+pushToken
            var url = "https://api.push.apple.com:443"+path
            
            const response = await fetch(url, {
                method: 'POST',
                body: null,
                headers: {
                    'apns-topic': 'com.colorfull.walletPass',
                    'apns-push-type': 'background',
                    'connection': 'keep-alive'
                }
            });

Got this error.

TypeError: fetch failed

Tried with Flutter like this.

var path = "/3/device/$pushToken";
        var url = "https://api.push.apple.com:443$path";
        Response resp = await post(Uri.parse(url), headers: {
          'apns-topic': 'com.colorfull.walletPass',
          'apns-push-type': 'background',
          'connection': 'keep-alive'
        }, body: {});

Got this error.

ClientException: Invalid request method, uri=https://api.push.apple.com/3/device/688a3d6bf83526f00f6fb3d8dd197578f665dea0eeba7aebbe68c61f06bad198

Obviously I'm doing it all wrong, but there is no clear guideline. How can I send APN??