Sometimes, Amazon may ask for the customer ID (CID) for you or someone testing / using your skill, in order to perform debugging or whitelisting.

The best way to do this is to send Amazon an utterance activity key for that user. With that, Amazon can derive the CID. Please follow instructions on How to get utterance activity keys.

Once you have the utterance activity key, the CID is the bold segment in the example below:

alexa.amazon.com/spa/index.html#settings/dialogs/AIKN8XKBNV6T8#1554911137444#AB72C64C86AW2#B0F00715602302CV