If you're using the in-browser client in Chrome, you can get the Telerik AppBuilder Helper for Chrome which enables Ajax CORS requests for the device simulator to spare you the CORS errors.
You can get it from here
. The team plans to bring more improvements in the Helper in the future, so keep an eye on it.
Share what you think about AppBuilder and Telerik Platform with us in our feedback portal, so we can become even better!