REST API to set security roles on service endpoints

I am trying to use this API in order to programmatically adjust the security role assignments on a service connection (called service endpoint by the API). I have the endpoint ID (resource ID), and I have the ID of the user (group, in this case) I want to add, but I cannot figure out what the scopeId field is supposed to be. This API is published but the documentation appears incomplete.

I have observed using my browser's developer console that my payload is the same using the 7.1 preview API, but the URL and API version is different via the ADO web UI, so it's not much help there.

Any help in this matter would be greatly appreciated.