"Wanna try anal?" That might work.
You shouldn't ask so directly. That'd be putting her on the spot and even if she would like to experiment, she'll probably say no (unless you're filming a porno). Go slooooooow. Touch her bumhole, make her ass a part of your sex routine. See how she responds (a fit of giggles and screeching is a good start) Make it feel good for her and it'll plant seeds in her mind. If she likes what you're doing, after a while (weeks) as you spoon her, try saying something like, "Imagine my cock is in your bum," and be GENTLE. Barely move your cock. Talk dirty to her. "God, your ass feels amazing around my cock." Make it a fantasy of hers and it's more likely to become a reality of yours.