Hi Zhifang,
AFAIK, due to the fact that we are not the network administrators, we are behind a http(s) proxy which captures all mentioned traffic to the Internet being itself who finally sends/gets the contents, making first a filter of the contents asked and evaluating if they are convenient for the environment we are in.
So, in any application who needs access to the Internet, it is mandatory to configure this proxy to access any content. I.e. configuring it on Internet Explorer (or whichever browser you use) under the label “Connections”à LAN Settings à Proxy Server.
With this firewall/proxy deployment, there is no machine except the proxy server who has http(s) access. While the traffic is sent via Chrome, we can access arcgisonline as the proxy is configured in the browser, but, we have sniffed http traffic by application and we noticed that the web app builder sends traffic by itself (the server mounted) so it cannot access the Internet, with the result of not being able to validate our application on arcgisonline.
Thanks. Best regards.