To collect a client's CVV number when sending them a credit card authorization form, make sure to toggle the option to collect that info to "Yes". To find this area, open up your credit card authorization form and click the pencil icon next to the question labeled "Please Enter Your Payment Information".
To view their CVV number along with the rest of their response, go to the client's profile in the "Contacts" area. Form responses are under the tab labeled "Forms".
You can also find form responses inside the form itself by clicking the tab labeled "Responses".