ChargeDesk only needs your Freshdesk support email address to setup a connection. This should be the email address which your customers send emails to, to create support tickets. If you do not know this email address, you can use support@{your_subdomain}.freshdesk.com where {your_subdomain} is the subdomain of your Freshdesk account.
To use ChargeDesk inside of Freshdesk you must install a ChargeDesk app to your Freshdesk account. Inside of Freshdesk you can install any of our apps (Stripe, Braintree or PayPal) and all data from your connected companies will come through the single app - it does not matter which app you install to your Freshdesk account. To install a Freshdesk app, go to Admin > Apps > Get More Apps > Search for "ChargeDesk".