How to set WindowsClientCredential.AllowedImpersonationLevel under WinRT?

I try to request a Sharepoint webservice from an MetroStyleApp. The Sharepoint returns an error (Exception from HRESULT: 0x80131904). After some googleing I found, that the problem is, that the credentials don't get passed to the SQL-Server. In an normal (Win32) Application I would set WindowsClientCredential.AllowedImpersonationLevel to Delegation. But there is no property like this under WinRT. How to solve this problem?

---