CRM Online: Plugin Registration fails because of a lack of prvReadServiceEndpoint privilege

I tried to register a CRM Plugin in my CRM Online (trial) instance using the PluginRegistration tool. The result is an error:

Principal user (Id=7557a920-d54f-e611-80da-fc15b4280c68, type=8) is missing prvReadServiceEndpoint privilege (Id=d86523d6-5946-4ddb-8176-4778116f8213).

How and where can the appropriate privileges be set ?

Thanks in advance.