You may Enable or Disable whichever custom headers you would like by providing a parameter name and its value in the app builder. Using
%DEVICEID% as the value will send up the device's unique identifier, and using
%DEVICENAME64% as the value will send the model name on Android, and something like "Gary's iPhone" on iOS.
Please note that custom headers are only added to the request for the original document request. Any CSS, JS, or image resources, or any other resources requested via AJAX will not include the custom headers in order to maintain fast performance.
In order to test the custom headers, you may click "View more devices" from your app's Private Management URL, and then enable the Network Intercept option on the bottom. This will show all network traffic your app is making, and you may inspect the request headers using the tool.