Self Check In

Where do I indicate "Self Check in" for a property?

You'd probably want to mention that in one of the fields under Guest Instructions, and potentially also use that in a Field Code in an automatic pre-arrival email sent to your guests.

It's also under the "Guest Instructions" section in your Property settings ("Check-In Type" has several options). That syncs to Airbnb if you are API integrated and I believe that is what tells Airbnb whether to show the self-check-in flag.