You can’t define something like that using a content-browser created enum from within c++. You need to create the enum in c++ or use a blueprint-based property.
For reference, this is how you do it in c++:
UENUM( BlueprintType )
enum class EUserDefinedENum : uint8
{
U_SomeValue,
U_SomeOtherValue
};
UPROPERTY( EditAnywhere, BlueprintReadOnly, Category = Classes )
EUserDefinedEnum MyEnum;