As I've said previously, none of the partners I've had have ever expressed a preference, BUT the following would have been effective on me at starting the conversation:
Her - "Hey, have you ever wondered what it would be like to be circumcised?" This said while pulling back my foreskin and admiring by bare glans, then stroking me with lube.
Her - "You know I love your cock, but I also think it would look amazing like this!" Again said while pulling my skin back, and giving me oral.