How can I represent an anonymous union in FFI?

A struct I'm wrapping has the following structure:

struct uade_notification {
    enum uade_notification_type type;
    union {
        char *msg;
        struct uade_notification_song_end song_end;
    };
};

I can create a union using FFI::Union of course, but I'm not sure how to represent that in a struct layout. Is assigning it an arbitrary name the right way to go?

Best,
Misty