you dont wanna be too forceful in asking her. she will feel pressured into doing it if you go up and ask her face to face. bring it up softly but make sure that she is totally 100% certain brofre you do it because the worst thing is doing something you dont want to.
you can slowly approach that area and judge how she reacts. if she doesn't mind you having your hand between her legs, then you can try between her legs inside her pants, if you doesn't mind that, then she probably won't mind being touched some more. and if you are unsure, getting her sexually excited by kissing, talking, touching, before hand can help. usually when someone's excited, they feel like engaging in more sexual activities. you can ask her if she thinks whatever you're doing feels good also, and go from there.