You can't. It doesn't matter if you mean a NowSupport ticket or a Customer Service ticket within your own system: if you create it for/on behave of the customer, you will have it visible for the customer. You should also ask yourself the question why you would want to hide it for them?
You don't explain the why, but it could get a huge breach of trust if they find out afterwards. Remember: the customer is the one with the contract (either with you, or with ServiceNow - depending on what you mean with CS case), you just work for them. Hiding is never a good thing.
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