var configurationServer = new TfsConfigurationServer(new Uri("https://xxxx"),
                                                new VssCredentials(new VssBasicCredential(
                    string.Empty,
                     "xxxxxx"
                 )));
configurationServer.EnsureAuthenticated();

var identityService = configurationServer.GetService<IIdentityManagementService>();

// Look up the user that we want to impersonate
TeamFoundationIdentity identity = identityService.ReadIdentity(
        IdentitySearchFactor.AccountName, "xx@xx.xxx", MembershipQuery.None, ReadIdentityOptions.None);

var impersonatedconfigurationServer = new TfsConfigurationServer(new Uri("https://xxxx"),
                                   new VssCredentials(new VssBasicCredential(
        string.Empty,
         "xxxxxx"
    )), identity.Descriptor);

impersonatedconfigurationServer.EnsureAuthenticated();