To change the sync function on an App Endpoint, please use the management API https://docs.couchbase.com/cloud/management-api-reference/index.html#tag/App-Endpoints/operation/putAccessFunction

Only /_roles, /_users and /_session are allowed through through the admin port 4985 in the connect tab on App Endpoints.

For the public port 4984, these are the supported endpoints https://docs.couchbase.com/cloud/app-services/references/rest_api_public.html