For a standard Guesty setup, your booking engine needs only your Stripe publishable key. It starts with pk_live.
Where to find it. In your Stripe dashboard, go to Developers, then API keys, and copy the key labeled Publishable key.
The publishable key is designed to be public, so it is safe to send in a normal email. It must come from the same Stripe account that is connected to your Guesty account.