Where do I find my event’s access code?

On the Sharing tab of your event dashboard; it is created with the event and is never shown to guests automatically.

It is on the Sharing tab, beside the shareable link. Every event gets one when it is created.

Guests are never sent it and it is never included in anything the guest side of the product shows — it is deliberately stripped out of everything a guest can fetch, so a curious guest cannot read it out of the page. Handing it out is yours to do, on a table card, in the invitation message, or on the poster.

Codes ignore capitals, so a guest typing it in lower case is fine. A code only ever opens the event it belongs to.