We are encountering an issue when trying to Get ARCustomer Details using an Optional Field as a filter. We have tried using the following syntax 

CustomerOptionalFieldValues.OptionalField eq 'IDCARD'

however we get the following error message : 

The child type 'CustomerOptionalFieldValues.OptionalField' in a cast was not an entity type. Casts can only be performed on entity types.

When we change the syntax to 

CustomerOptionalFieldValues /OptionalField eq 'IDCARD' we get the following error message : 

The parent value for a property access of a property 'OptionalField' is not a single value. Property access can only be applied to a single value

Is there any way possible to use an Optional Field as a filter ? Any assistance would be greatly appreciated.

Best regards,

Mark