Unable to get customers from Service Desk Cloud REST API endpoint


I’m trying to automate issue creating for our company. For this I’m using the Service Desk Cloud REST API.
Using OAuth2 I am able to call many other endpoints without, but the experimental GET /rest/servicedeskapi/servicedesk/{serviceDeskId}/customer endpoint returns:

{ "errorMessage": "The action performed required a logged in user. Please log in and try again.", "i18nErrorMessage": { "i18nKey": "sd.admin.servicedesk.error.authenticated.user.required" } }

I want to use this endpoint to get the customerId that corresponds to a certain email, to be able to create and raise issues on the customer’s behalf.

Does anybody know why authentication works with endpoints such as POST /rest/servicedeskapi/customer, that has ADMIN scope, but doesn’t with this one with READ scope?