Depending on the CREDENTIAL_PROVIDER_USAGE_SCENARIO passed into ICredentialProvider::SetUsageScenario
ICredentialProviderCredential::GetSerialization
must return the serialized form of one of the following structs
KERB_CERTIFICATE_LOGON
KERB_CERTIFICATE_UNLOCK_LOGON
Searching the web for each of the structures should turn up code for serializing them.