Tell her best friend and ask her to tell your wife.
No, of course not! We aren’t psychic you know! What did you think, you could play some subliminal tape saying ‘your husband wants it in the arse’ and she’d suddenly buy a huge strap on and fuck you with it?!
TALK TO HER – start slow, be romantic, make her feel loved and cherished and very slowly introduce some kinky things like vibrators and butt plugs, and IF, IF she gets to like that you might just find her willing to explore pegging.